Chapter 4: Generators

In Chapter 2, we identified two key drawbacks to expressing async flow control with callbacks:

• Callback-based async doesn't fit how our brain plans out steps of a task.
• Callbacks aren't trustable or composable because of inversion of control.

In Chapter 3, we detailed how Promises uninvert the inversion of control of callbacks, restoring trustability/composability.

Now we turn our attention to expressing async flow control in a sequential, synchronous-looking fashion. The "magic" that makes it possible is ES6 generators.

Breaking Run-to-Completion

In Chapter 1, we explained an expectation that JS developers almost universally rely on in their code: once a function starts executing, it runs until it completes, and no other code can interrupt and run in between.

As bizarre as it may seem, ES6 introduces a new type of function that does not behave with the run-to-completion behavior. This new type of function is called a "generator."

To understand the implications, let's consider this example:

var x = 1;

function foo() {
    x++;
    bar();                // <-- what about this line?
    console.log( "x:", x );
}

function bar() {
    x++;
}

foo();                    // x: 3

In this example, we know for sure that bar() runs in between x++ and console.log(x). But what if bar() wasn't there? Obviously the result would be 2 instead of 3.

Now let's twist your brain. What if bar() wasn't present, but it could still somehow run between the x++ and console.log(x) statements? How would that be possible?

In preemptive multithreaded languages, it would essentially be possible for bar() to "interrupt" and run at exactly the right moment between those two statements. But JS is not preemptive, nor is it (currently) multithreaded. And yet, a cooperative form of this "interruption" (concurrency) is possible, if foo() itself could somehow indicate a "pause" at that part in the code.

Note: I use the word "cooperative" not only because of the connection to classical concurrency terminology (see Chapter 1), but because as you'll see in the next snippet, the ES6 syntax for indicating a pause point in code is yield -- suggesting a politely cooperative yielding of control.

Here's the ES6 code to accomplish such cooperative concurrency:

var x = 1;

function *foo() {
    x++;
    yield; // pause!
    console.log( "x:", x );
}

function bar() {
    x++;
}

Note: You will likely see most other JS documentation/code that will format a generator declaration as function* foo() { .. } instead of as I've done here with function *foo() { .. } -- the only difference being the stylistic positioning of the *. The two forms are functionally/syntactically identical, as is a third function*foo() { .. } (no space) form. There are arguments for both styles, but I basically prefer function *foo.. because it then matches when I reference a generator in writing with *foo(). If I said only foo(), you wouldn't know as clearly if I was talking about a generator or a regular function. It's purely a stylistic preference.

Now, how can we run the code in that previous snippet such that bar() executes at the point of the yield inside of *foo()?

// construct an iterator `it` to control the generator
var it = foo();

// start `foo()` here!
it.next();
x;                        // 2
bar();
x;                        // 3
it.next();                // x: 3

OK, there's quite a bit of new and potentially confusing stuff in those two code snippets, so we've got plenty to wade through. But before we explain the different mechanics/syntax with ES6 generators, let's walk through the behavior flow:

1. The it = foo() operation does not execute the *foo() generator yet, but it merely constructs an iterator that will control its execution. More on iterators in a bit.
2. The first it.next() starts the *foo() generator, and runs the x++ on the first line of *foo().
3. *foo() pauses at the yield statement, at which point that first it.next() call finishes. At the moment, *foo() is still running and active, but it's in a paused state.
4. We inspect the value of x, and it's now 2.
5. We call bar(), which increments x again with x++.
6. We inspect the value of x again, and it's now 3.
7. The final it.next() call resumes the *foo() generator from where it was paused, and runs the console.log(..) statement, which uses the current value of x of 3.

Clearly, foo() started, but did not run-to-completion -- it paused at the yield. We resumed foo() later, and let it finish, but that wasn't even required.

So, a generator is a special kind of function that can start and stop one or more times, and doesn't necessarily ever have to finish. While it won't be terribly obvious yet why that's so powerful, as we go throughout the rest of this chapter, that will be one of the fundamental building blocks we use to construct generators-as-async-flow-control as a pattern for our code.

Input and Output

A generator function is a special function with the new processing model we just alluded to. But it's still a function, which means it still has some basic tenets that haven't changed -- namely, that it still accepts arguments (aka "input"), and that it can still return a value (aka "output"):

function *foo(x,y) {
    return x * y;
}

var it = foo( 6, 7 );

var res = it.next();

res.value;        // 42

We pass in the arguments 6 and 7 to *foo(..) as the parameters x and y, respectively. And *foo(..) returns the value 42 back to the calling code.

We now see a difference with how the generator is invoked compared to a normal function. foo(6,7) obviously looks familiar. But subtly, the *foo(..) generator hasn't actually run yet as it would have with a function.

Instead, we're just creating an iterator object, which we assign to the variable it, to control the *foo(..) generator. Then we call it.next(), which instructs the *foo(..) generator to advance from its current location, stopping either at the next yield or end of the generator.

The result of that next(..) call is an object with a value property on it holding whatever value (if anything) was returned from *foo(..). In other words, yield caused a value to be sent out from the generator during the middle of its execution, kind of like an intermediate return.

Again, it won't be obvious yet why we need this whole indirect iterator object to control the generator. We'll get there, I promise.

Iteration Messaging

In addition to generators accepting arguments and having return values, there's even more powerful and compelling input/output messaging capability built into them, via yield and next(..).

Consider:

function *foo(x) {
    var y = x * (yield);
    return y;
}

var it = foo( 6 );

// start `foo(..)`
it.next();

var res = it.next( 7 );

res.value;        // 42

First, we pass in 6 as the parameter x. Then we call it.next(), and it starts up *foo(..).

Inside *foo(..), the var y = x .. statement starts to be processed, but then it runs across a yield expression. At that point, it pauses *foo(..) (in the middle of the assignment statement!), and essentially requests the calling code to provide a result value for the yield expression. Next, we call it.next( 7 ), which is passing the 7 value back in to be that result of the paused yield expression.

So, at this point, the assignment statement is essentially var y = 6 * 7. Now, return y returns that 42 value back as the result of the it.next( 7 ) call.

Notice something very important but also easily confusing, even to seasoned JS developers: depending on your perspective, there's a mismatch between the yield and the next(..) call. In general, you're going to have one more next(..) call than you have yield statements -- the preceding snippet has one yield and two next(..) calls.

Why the mismatch?

Because the first next(..) always starts a generator, and runs to the first yield. But it's the second next(..) call that fulfills the first paused yield expression, and the third next(..) would fulfill the second yield, and so on.

Tale of Two Questions

Actually, which code you're thinking about primarily will affect whether there's a perceived mismatch or not.

Consider only the generator code:

var y = x * (yield);
return y;

This first yield is basically asking a question: "What value should I insert here?"

Who's going to answer that question? Well, the first next() has already run to get the generator up to this point, so obviously it can't answer the question. So, the second next(..) call must answer the question posed by the first yield.

See the mismatch -- second-to-first?

But let's flip our perspective. Let's look at it not from the generator's point of view, but from the iterator's point of view.

To properly illustrate this perspective, we also need to explain that messages can go in both directions -- yield .. as an expression can send out messages in response to next(..) calls, and next(..) can send values to a paused yield expression. Consider this slightly adjusted code:

function *foo(x) {
    var y = x * (yield "Hello");    // <-- yield a value!
    return y;
}

var it = foo( 6 );

var res = it.next();    // first `next()`, don't pass anything
res.value;                // "Hello"

res = it.next( 7 );        // pass `7` to waiting `yield`
res.value;                // 42

yield .. and next(..) pair together as a two-way message passing system during the execution of the generator.

So, looking only at the iterator code:

var res = it.next();    // first `next()`, don't pass anything
res.value;                // "Hello"

res = it.next( 7 );        // pass `7` to waiting `yield`
res.value;                // 42

Note: We don't pass a value to the first next() call, and that's on purpose. Only a paused yield could accept such a value passed by a next(..), and at the beginning of the generator when we call the first next(), there is no paused yield to accept such a value. The specification and all compliant browsers just silently discard anything passed to the first next(). It's still a bad idea to pass a value, as you're just creating silently "failing" code that's confusing. So, always start a generator with an argument-free next().

The first next() call (with nothing passed to it) is basically asking a question: "What next value does the *foo(..) generator have to give me?" And who answers this question? The first yield "hello" expression.

See? No mismatch there.

Depending on who you think about asking the question, there is either a mismatch between the yield and next(..) calls, or not.

But wait! There's still an extra next() compared to the number of yield statements. So, that final it.next(7) call is again asking the question about what next value the generator will produce. But there's no more yield statements left to answer, is there? So who answers?

The return statement answers the question!

And if there is no return in your generator -- return is certainly not any more required in generators than in regular functions -- there's always an assumed/implicit return; (aka return undefined;), which serves the purpose of default answering the question posed by the final it.next(7) call.

These questions and answers -- the two-way message passing with yield and next(..) -- are quite powerful, but it's not obvious at all how these mechanisms are connected to async flow control. We're getting there!

Multiple Iterators

It may appear from the syntactic usage that when you use an iterator to control a generator, you're controlling the declared generator function itself. But there's a subtlety that easy to miss: each time you construct an iterator, you are implicitly constructing an instance of the generator which that iterator will control.

You can have multiple instances of the same generator running at the same time, and they can even interact:

function *foo() {
    var x = yield 2;
    z++;
    var y = yield (x * z);
    console.log( x, y, z );
}

var z = 1;

var it1 = foo();
var it2 = foo();

var val1 = it1.next().value;            // 2 <-- yield 2
var val2 = it2.next().value;            // 2 <-- yield 2

val1 = it1.next( val2 * 10 ).value;        // 40  <-- x:20,  z:2
val2 = it2.next( val1 * 5 ).value;        // 600 <-- x:200, z:3

it1.next( val2 / 2 );                    // y:300
                                        // 20 300 3
it2.next( val1 / 4 );                    // y:10
                                        // 200 10 3

Warning: The most common usage of multiple instances of the same generator running concurrently is not such interactions, but when the generator is producing its own values without input, perhaps from some independently connected resource. We'll talk more about value production in the next section.

Let's briefly walk through the processing:

1. Both instances of *foo() are started at the same time, and both next() calls reveal a value of 2 from the yield 2 statements, respectively.
2. val2 * 10 is 2 * 10, which is sent into the first generator instance it1, so that x gets value 20. z is incremented from 1 to 2, and then 20 * 2 is yielded out, setting val1 to 40.
3. val1 * 5 is 40 * 5, which is sent into the second generator instance it2, so that x gets value 200. z is incremented again, from 2 to 3, and then 200 * 3 is yielded out, setting val2 to 600.
4. val2 / 2 is 600 / 2, which is sent into the first generator instance it1, so that y gets value 300, then printing out 20 300 3 for its x y z values, respectively.
5. val1 / 4 is 40 / 4, which is sent into the second generator instance it2, so that y gets value 10, then printing out 200 10 3 for its x y z values, respectively.

That's a "fun" example to run through in your mind. Did you keep it straight?

Interleaving

Recall this scenario from the "Run-to-completion" section of Chapter 1:

var a = 1;
var b = 2;

function foo() {
    a++;
    b = b * a;
    a = b + 3;
}

function bar() {
    b--;
    a = 8 + b;
    b = a * 2;
}

With normal JS functions, of course either foo() can run completely first, or bar() can run completely first, but foo() cannot interleave its individual statements with bar(). So, there are only two possible outcomes to the preceding program.

However, with generators, clearly interleaving (even in the middle of statements!) is possible:

var a = 1;
var b = 2;

function *foo() {
    a++;
    yield;
    b = b * a;
    a = (yield b) + 3;
}

function *bar() {
    b--;
    yield;
    a = (yield 8) + b;
    b = a * (yield 2);
}

Depending on what respective order the iterators controlling *foo() and *bar() are called, the preceding program could produce several different results. In other words, we can actually illustrate (in a sort of fake-ish way) the theoretical "threaded race conditions" circumstances discussed in Chapter 1, by interleaving the two generator interations over the same shared variables.

First, let's make a helper called step(..) that controls an iterator:

function step(gen) {
    var it = gen();
    var last;

    return function() {
        // whatever is `yield`ed out, just
        // send it right back in the next time!
        last = it.next( last ).value;
    };
}

step(..) initializes a generator to create its it iterator, then returns a function which, when called, advances the iterator by one step. Additionally, the previously yielded out value is sent right back in at the next step. So, yield 8 will just become 8 and yield b will just be b (whatever it was at the time of yield).

Now, just for fun, let's experiment to see the effects of interleaving these different chunks of *foo() and *bar(). We'll start with the boring base case, making sure *foo() totally finishes before *bar() (just like we did in Chapter 1):

// make sure to reset `a` and `b`
a = 1;
b = 2;

var s1 = step( foo );
var s2 = step( bar );

// run `*foo()` completely first
s1();
s1();
s1();

// now run `*bar()`
s2();
s2();
s2();
s2();

console.log( a, b );    // 11 22

The end result is 11 and 22, just as it was in the Chapter 1 version. Now let's mix up the interleaving ordering and see how it changes the final values of a and b:

// make sure to reset `a` and `b`
a = 1;
b = 2;

var s1 = step( foo );
var s2 = step( bar );

s2();        // b--;
s2();        // yield 8
s1();        // a++;
s2();        // a = 8 + b;
            // yield 2
s1();        // b = b * a;
            // yield b
s1();        // a = b + 3;
s2();        // b = a * 2;

Before I tell you the results, can you figure out what a and b are after the preceding program? No cheating!

console.log( a, b );    // 12 18

Note: As an exercise for the reader, try to see how many other combinations of results you can get back rearranging the order of the s1() and s2() calls. Don't forget you'll always need three s1() calls and four s2() calls. Recall the discussion earlier about matching next() with yield for the reasons why.

You almost certainly won't want to intentionally create this level of interleaving confusion, as it creates incredibly difficult to understand code. But the exercise is interesting and instructive to understand more about how multiple generators can run concurrently in the same shared scope, because there will be places where this capability is quite useful.

We'll discuss generator concurrency in more detail at the end of this chapter.

Generator'ing Values

In the previous section, we mentioned an interesting use for generators, as a way to produce values. This is not the main focus in this chapter, but we'd be remiss if we didn't cover the basics, especially because this use case is essentially the origin of the name: generators.

We're going to take a slight diversion into the topic of iterators for a bit, but we'll circle back to how they relate to generators and using a generator to generate values.

Producers and Iterators

Imagine you're producing a series of values where each value has a definable relationship to the previous value. To do this, you're going to need a stateful producer that remembers the last value it gave out.

You can implement something like that straightforwardly using a function closure (see the Scope & Closures title of this series):

var gimmeSomething = (function(){
    var nextVal;

    return function(){
        if (nextVal === undefined) {
            nextVal = 1;
        }
        else {
            nextVal = (3 * nextVal) + 6;
        }

        return nextVal;
    };
})();

gimmeSomething();        // 1
gimmeSomething();        // 9
gimmeSomething();        // 33
gimmeSomething();        // 105

Note: The nextVal computation logic here could have been simplified, but conceptually, we don't want to calculate the next value (aka nextVal) until the next gimmeSomething() call happens, because in general that could be a resource-leaky design for producers of more persistent or resource-limited values than simple numbers.

Generating an arbitrary number series isn't a terribly realistic example. But what if you were generating records from a data source? You could imagine much the same code.

In fact, this task is a very common design pattern, usually solved by iterators. An iterator is a well-defined interface for stepping through a series of values from a producer. The JS interface for iterators, as it is in most languages, is to call next() each time you want the next value from the producer.

We could implement the standard iterator interface for our number series producer:

var something = (function(){
    var nextVal;

    return {
        // needed for `for..of` loops
        [Symbol.iterator]: function(){ return this; },

        // standard iterator interface method
        next: function(){
            if (nextVal === undefined) {
                nextVal = 1;
            }
            else {
                nextVal = (3 * nextVal) + 6;
            }

            return { done:false, value:nextVal };
        }
    };
})();

something.next().value;        // 1
something.next().value;        // 9
something.next().value;        // 33
something.next().value;        // 105

Note: We'll explain why we need the [Symbol.iterator]: .. part of this code snippet in the "Iterables" section. Syntactically though, two ES6 features are at play. First, the [ .. ] syntax is called a computed property name (see the this & Object Prototypes title of this series). It's a way in an object literal definition to specify an expression and use the result of that expression as the name for the property. Next, Symbol.iterator is one of ES6's predefined special Symbol values (see the ES6 & Beyond title of this book series).

The next() call returns an object with two properties: done is a boolean value signaling the iterator's complete status; value holds the iteration value.

ES6 also adds the for..of loop, which means that a standard iterator can automatically be consumed with native loop syntax:

for (var v of something) {
    console.log( v );

    // don't let the loop run forever!
    if (v > 500) {
        break;
    }
}
// 1 9 33 105 321 969

Note: Because our something iterator always returns done:false, this for..of loop would run forever, which is why we put the break conditional in. It's totally OK for iterators to be never-ending, but there are also cases where the iterator will run over a finite set of values and eventually return a done:true.

The for..of loop automatically calls next() for each iteration -- it doesn't pass any values in to the next() -- and it will automatically terminate on receiving a done:true. It's quite handy for looping over a set of data.

Of course, you could manually loop over iterators, calling next() and checking for the done:true condition to know when to stop:

for (
    var ret;
    (ret = something.next()) && !ret.done;
) {
    console.log( ret.value );

    // don't let the loop run forever!
    if (ret.value > 500) {
        break;
    }
}
// 1 9 33 105 321 969

Note: This manual for approach is certainly uglier than the ES6 for..of loop syntax, but its advantage is that it affords you the opportunity to pass in values to the next(..) calls if necessary.

In addition to making your own iterators, many built-in data structures in JS (as of ES6), like arrays, also have default iterators:

var a = [1,3,5,7,9];

for (var v of a) {
    console.log( v );
}
// 1 3 5 7 9

The for..of loop asks a for its iterator, and automatically uses it to iterate over a's values.

Note: It may seem a strange omission by ES6, but regular objects intentionally do not come with a default iterator the way arrays do. The reasons go deeper than we will cover here. If all you want is to iterate over the properties of an object (with no particular guarantee of ordering), Object.keys(..) returns an array, which can then be used like for (var k of Object.keys(obj)) { ... Such a for..of loop over an object's keys would be similar to a for..in loop, except that Object.keys(..) does not include properties from the [[Prototype]] chain while for..in does (see the this & Object Prototypes title of this series).

Iterables

The something object in our running example is called an iterator, as it has the next() method on its interface. But a closely related term is iterable, which is an object that contains an iterator that can iterate over its values.

As of ES6, the way to retrieve an iterator from an iterable is that the iterable must have a function on it, with the name being the special ES6 symbol value Symbol.iterator. When this function is called, it returns an iterator. Though not required, generally each call should return a fresh new iterator.

a in the previous snippet is an iterable. The for..of loop automatically calls its Symbol.iterator function to construct an iterator. But we could of course call the function manually, and use the iterator it returns:

var a = [1,3,5,7,9];

var it = a[Symbol.iterator]();

it.next().value;    // 1
it.next().value;    // 3
it.next().value;    // 5
..

In the previous code listing that defined something, you may have noticed this line:

[Symbol.iterator]: function(){ return this; }

That little bit of confusing code is making the something value -- the interface of the something iterator -- also an iterable; it's now both an iterable and an iterator. Then, we pass something to the for..of loop:

for (var v of something) {
    ..
}

The for..of loop expects something to be an iterable, so it looks for and calls its Symbol.iterator function. We defined that function to simply return this, so it just gives itself back, and the for..of loop is none the wiser.

Generator Iterator

Let's turn our attention back to generators, in the context of iterators. A generator can be treated as a producer of values that we extract one at a time through an iterator interface's next() calls.

So, a generator itself is not technically an iterable, though it's very similar -- when you execute the generator, you get an iterator back:

function *foo(){ .. }

var it = foo();

We can implement the something infinite number series producer from earlier with a generator, like this:

function *something() {
    var nextVal;

    while (true) {
        if (nextVal === undefined) {
            nextVal = 1;
        }
        else {
            nextVal = (3 * nextVal) + 6;
        }

        yield nextVal;
    }
}

Note: A while..true loop would normally be a very bad thing to include in a real JS program, at least if it doesn't have a break or return in it, as it would likely run forever, synchronously, and block/lock-up the browser UI. However, in a generator, such a loop is generally totally OK if it has a yield in it, as the generator will pause at each iteration, yielding back to the main program and/or to the event loop queue. To put it glibly, "generators put the while..true back in JS programming!"

That's a fair bit cleaner and simpler, right? Because the generator pauses at each yield, the state (scope) of the function *something() is kept around, meaning there's no need for the closure boilerplate to preserve variable state across calls.

Not only is it simpler code -- we don't have to make our own iterator interface -- it actually is more reason-able code, because it more clearly expresses the intent. For example, the while..true loop tells us the generator is intended to run forever -- to keep generating values as long as we keep asking for them.

And now we can use our shiny new *something() generator with a for..of loop, and you'll see it works basically identically:

for (var v of something()) {
    console.log( v );

    // don't let the loop run forever!
    if (v > 500) {
        break;
    }
}
// 1 9 33 105 321 969

But don't skip over for (var v of something()) ..! We didn't just reference something as a value like in earlier examples, but instead called the *something() generator to get its iterator for the for..of loop to use.

If you're paying close attention, two questions may arise from this interaction between the generator and the loop:

• Why couldn't we say for (var v of something) ..? Because something here is a generator, which is not an iterable. We have to call something() to construct a producer for the for..of loop to iterate over.
• The something() call produces an iterator, but the for..of loop wants an iterable, right? Yep. The generator's iterator also has a Symbol.iterator function on it, which basically does a return this, just like the something iterable we defined earlier. In other words, a generator's iterator is also an iterable!

Stopping the Generator

In the previous example, it would appear the iterator instance for the *something() generator was basically left in a suspended state forever after the break in the loop was called.

But there's a hidden behavior that takes care of that for you. "Abnormal completion" (i.e., "early termination") of the for..of loop -- generally caused by a break, return, or an uncaught exception -- sends a signal to the generator's iterator for it to terminate.

Note: Technically, the for..of loop also sends this signal to the iterator at the normal completion of the loop. For a generator, that's essentially a moot operation, as the generator's iterator had to complete first so the for..of loop completed. However, custom iterators might desire to receive this additional signal from for..of loop consumers.

While a for..of loop will automatically send this signal, you may wish to send the signal manually to an iterator; you do this by calling return(..).

If you specify a try..finally clause inside the generator, it will always be run even when the generator is externally completed. This is useful if you need to clean up resources (database connections, etc.):

function *something() {
    try {
        var nextVal;

        while (true) {
            if (nextVal === undefined) {
                nextVal = 1;
            }
            else {
                nextVal = (3 * nextVal) + 6;
            }

            yield nextVal;
        }
    }
    // cleanup clause
    finally {
        console.log( "cleaning up!" );
    }
}

The earlier example with break in the for..of loop will trigger the finally clause. But you could instead manually terminate the generator's iterator instance from the outside with return(..):

var it = something();
for (var v of it) {
    console.log( v );

    // don't let the loop run forever!
    if (v > 500) {
        console.log(
            // complete the generator's iterator
            it.return( "Hello World" ).value
        );
        // no `break` needed here
    }
}
// 1 9 33 105 321 969
// cleaning up!
// Hello World

When we call it.return(..), it immediately terminates the generator, which of course runs the finally clause. Also, it sets the returned value to whatever you passed in to return(..), which is how "Hello World" comes right back out. We also don't need to include a break now because the generator's iterator is set to done:true, so the for..of loop will terminate on its next iteration.

Generators owe their namesake mostly to this consuming produced values use. But again, that's just one of the uses for generators, and frankly not even the main one we're concerned with in the context of this book.

But now that we more fully understand some of the mechanics of how they work, we can next turn our attention to how generators apply to async concurrency.

Iterating Generators Asynchronously

What do generators have to do with async coding patterns, fixing problems with callbacks, and the like? Let's get to answering that important question.

We should revisit one of our scenarios from Chapter 3. Let's recall the callback approach:

function foo(x,y,cb) {
    ajax(
        "http://some.url.1/?x=" + x + "&y=" + y,
        cb
    );
}

foo( 11, 31, function(err,text) {
    if (err) {
        console.error( err );
    }
    else {
        console.log( text );
    }
} );

If we wanted to express this same task flow control with a generator, we could do:

function foo(x,y) {
    ajax(
        "http://some.url.1/?x=" + x + "&y=" + y,
        function(err,data){
            if (err) {
                // throw an error into `*main()`
                it.throw( err );
            }
            else {
                // resume `*main()` with received `data`
                it.next( data );
            }
        }
    );
}

function *main() {
    try {
        var text = yield foo( 11, 31 );
        console.log( text );
    }
    catch (err) {
        console.error( err );
    }
}

var it = main();

// start it all up!
it.next();

At first glance, this snippet is longer, and perhaps a little more complex looking, than the callback snippet before it. But don't let that impression get you off track. The generator snippet is actually much better! But there's a lot going on for us to explain.

First, let's look at this part of the code, which is the most important:

var text = yield foo( 11, 31 );
console.log( text );

Think about how that code works for a moment. We're calling a normal function foo(..) and we're apparently able to get back the text from the Ajax call, even though it's asynchronous.

How is that possible? If you recall the beginning of Chapter 1, we had almost identical code:

var data = ajax( "..url 1.." );
console.log( data );

And that code didn't work! Can you spot the difference? It's the yield used in a generator.

That's the magic! That's what allows us to have what appears to be blocking, synchronous code, but it doesn't actually block the whole program; it only pauses/blocks the code in the generator itself.

In yield foo(11,31), first the foo(11,31) call is made, which returns nothing (aka undefined), so we're making a call to request data, but we're actually then doing yield undefined. That's OK, because the code is not currently relying on a yielded value to do anything interesting. We'll revisit this point later in the chapter.

We're not using yield in a message passing sense here, only in a flow control sense to pause/block. Actually, it will have message passing, but only in one direction, after the generator is resumed.

So, the generator pauses at the yield, essentially asking the question, "what value should I return to assign to the variable text?" Who's going to answer that question?

Look at foo(..). If the Ajax request is successful, we call:

it.next( data );

That's resuming the generator with the response data, which means that our paused yield expression receives that value directly, and then as it restarts the generator code, that value gets assigned to the local variable text.

Pretty cool, huh?

Take a step back and consider the implications. We have totally synchronous-looking code inside the generator (other than the yield keyword itself), but hidden behind the scenes, inside of foo(..), the operations can complete asynchronously.

That's huge! That's a nearly perfect solution to our previously stated problem with callbacks not being able to express asynchrony in a sequential, synchronous fashion that our brains can relate to.

In essence, we are abstracting the asynchrony away as an implementation detail, so that we can reason synchronously/sequentially about our flow control: "Make an Ajax request, and when it finishes print out the response." And of course, we just expressed two steps in the flow control, but this same capabililty extends without bounds, to let us express however many steps we need to.

Tip: This is such an important realization, just go back and read the last three paragraphs again to let it sink in!

Synchronous Error Handling

But the preceding generator code has even more goodness to yield to us. Let's turn our attention to the try..catch inside the generator:

try {
    var text = yield foo( 11, 31 );
    console.log( text );
}
catch (err) {
    console.error( err );
}

How does this work? The foo(..) call is asynchronously completing, and doesn't try..catch fail to catch asynchronous errors, as we looked at in Chapter 3?

We already saw how the yield lets the assignment statement pause to wait for foo(..) to finish, so that the completed response can be assigned to text. The awesome part is that this yield pausing also allows the generator to catch an error. We throw that error into the generator with this part of the earlier code listing:

if (err) {
    // throw an error into `*main()`
    it.throw( err );
}

The yield-pause nature of generators means that not only do we get synchronous-looking return values from async function calls, but we can also synchronously catch errors from those async function calls!

So we've seen we can throw errors into a generator, but what about throwing errors out of a generator? Exactly as you'd expect:

function *main() {
    var x = yield "Hello World";

    yield x.toLowerCase();    // cause an exception!
}

var it = main();

it.next().value;            // Hello World

try {
    it.next( 42 );
}
catch (err) {
    console.error( err );    // TypeError
}

Of course, we could have manually thrown an error with throw .. instead of causing an exception.

We can even catch the same error that we throw(..) into the generator, essentially giving the generator a chance to handle it but if it doesn't, the iterator code must handle it:

function *main() {
    var x = yield "Hello World";

    // never gets here
    console.log( x );
}

var it = main();

it.next();

try {
    // will `*main()` handle this error? we'll see!
    it.throw( "Oops" );
}
catch (err) {
    // nope, didn't handle it!
    console.error( err );            // Oops
}

Synchronous-looking error handling (via try..catch) with async code is a huge win for readability and reason-ability.

Generators + Promises

In our previous discussion, we showed how generators can be iterated asynchronously, which is a huge step forward in sequential reason-ability over the spaghetti mess of callbacks. But we lost something very important: the trustability and composability of Promises (see Chapter 3)!

Don't worry -- we can get that back. The best of all worlds in ES6 is to combine generators (synchronous-looking async code) with Promises (trustable and composable).

But how?

Recall from Chapter 3 the Promise-based approach to our running Ajax example:

function foo(x,y) {
    return request(
        "http://some.url.1/?x=" + x + "&y=" + y
    );
}

foo( 11, 31 )
.then(
    function(text){
        console.log( text );
    },
    function(err){
        console.error( err );
    }
);

In our earlier generator code for the running Ajax example, foo(..) returned nothing (undefined), and our iterator control code didn't care about that yielded value.

But here the Promise-aware foo(..) returns a promise after making the Ajax call. That suggests that we could construct a promise with foo(..) and then yield it from the generator, and then the iterator control code would receive that promise.

But what should the iterator do with the promise?

It should listen for the promise to resolve (fulfillment or rejection), and then either resume the generator with the fulfillment message or throw an error into the generator with the rejection reason.

Let me repeat that, because it's so important. The natural way to get the most out of Promises and generators is to yield a Promise, and wire that Promise to control the generator's iterator.

Let's give it a try! First, we'll put the Promise-aware foo(..) together with the generator *main():

function foo(x,y) {
    return request(
        "http://some.url.1/?x=" + x + "&y=" + y
    );
}

function *main() {
    try {
        var text = yield foo( 11, 31 );
        console.log( text );
    }
    catch (err) {
        console.error( err );
    }
}

The most powerful revelation in this refactor is that the code inside *main() did not have to change at all! Inside the generator, whatever values are yielded out is just an opaque implementation detail, so we're not even aware it's happening, nor do we need to worry about it.

But how are we going to run *main() now? We still have some of the implementation plumbing work to do, to receive and wire up the yielded promise so that it resumes the generator upon resolution. We'll start by trying that manually:

var it = main();

var p = it.next().value;

// wait for the `p` promise to resolve
p.then(
    function(text){
        it.next( text );
    },
    function(err){
        it.throw( err );
    }
);

Actually, that wasn't so painful at all, was it?

This snippet should look very similar to what we did earlier with the manually wired generator controlled by the error-first callback. Instead of an if (err) { it.throw.., the promise already splits fulfillment (success) and rejection (failure) for us, but otherwise the iterator control is identical.

Now, we've glossed over some important details.

Most importantly, we took advantage of the fact that we knew that *main() only had one Promise-aware step in it. What if we wanted to be able to Promise-drive a generator no matter how many steps it has? We certainly don't want to manually write out the Promise chain differently for each generator! What would be much nicer is if there was a way to repeat (aka "loop" over) the iteration control, and each time a Promise comes out, wait on its resolution before continuing.

Also, what if the generator throws out an error (intentionally or accidentally) during the it.next(..) call? Should we quit, or should we catch it and send it right back in? Similarly, what if we it.throw(..) a Promise rejection into the generator, but it's not handled, and comes right back out?

Promise-Aware Generator Runner

The more you start to explore this path, the more you realize, "wow, it'd be great if there was just some utility to do it for me." And you're absolutely correct. This is such an important pattern, and you don't want to get it wrong (or exhaust yourself repeating it over and over), so your best bet is to use a utility that is specifically designed to run Promise-yielding generators in the manner we've illustrated.

Several Promise abstraction libraries provide just such a utility, including my asynquence library and its runner(..), which will be discussed in Appendix A of this book.

But for the sake of learning and illustration, let's just define our own standalone utility that we'll call run(..):

// thanks to Benjamin Gruenbaum (@benjamingr on GitHub) for
// big improvements here!
function run(gen) {
    var args = [].slice.call( arguments, 1), it;

    // initialize the generator in the current context
    it = gen.apply( this, args );

    // return a promise for the generator completing
    return Promise.resolve()
        .then( function handleNext(value){
            // run to the next yielded value
            var next = it.next( value );

            return (function handleResult(next){
                // generator has completed running?
                if (next.done) {
                    return next.value;
                }
                // otherwise keep going
                else {
                    return Promise.resolve( next.value )
                        .then(
                            // resume the async loop on
                            // success, sending the resolved
                            // value back into the generator
                            handleNext,

                            // if `value` is a rejected
                            // promise, propagate error back
                            // into the generator for its own
                            // error handling
                            function handleErr(err) {
                                return Promise.resolve(
                                    it.throw( err )
                                )
                                .then( handleResult );
                            }
                        );
                }
            })(next);
        } );
}

As you can see, it's a quite a bit more complex than you'd probably want to author yourself, and you especially wouldn't want to repeat this code for each generator you use. So, a utility/library helper is definitely the way to go. Nevertheless, I encourage you to spend a few minutes studying that code listing to get a better sense of how to manage the generator+Promise negotiation.

How would you use run(..) with *main() in our running Ajax example?

function *main() {
    // ..
}

run( main );

That's it! The way we wired run(..), it will automatically advance the generator you pass to it, asynchronously until completion.

Note: The run(..) we defined returns a promise which is wired to resolve once the generator is complete, or receive an uncaught exception if the generator doesn't handle it. We don't show that capability here, but we'll come back to it later in the chapter.

ES7: async and await?

The preceding pattern -- generators yielding Promises that then control the generator's iterator to advance it to completion -- is such a powerful and useful approach, it would be nicer if we could do it without the clutter of the library utility helper (aka run(..)).

There's probably good news on that front. At the time of this writing, there's early but strong support for a proposal for more syntactic addition in this realm for the post-ES6, ES7-ish timeframe. Obviously, it's too early to guarantee the details, but there's a pretty decent chance it will shake out similar to the following:

function foo(x,y) {
    return request(
        "http://some.url.1/?x=" + x + "&y=" + y
    );
}

async function main() {
    try {
        var text = await foo( 11, 31 );
        console.log( text );
    }
    catch (err) {
        console.error( err );
    }
}

main();

As you can see, there's no run(..) call (meaning no need for a library utility!) to invoke and drive main() -- it's just called as a normal function. Also, main() isn't declared as a generator function anymore; it's a new kind of function: async function. And finally, instead of yielding a Promise, we await for it to resolve.

The async function automatically knows what to do if you await a Promise -- it will pause the function (just like with generators) until the Promise resolves. We didn't illustrate it in this snippet, but calling an async function like main() automatically returns a promise that's resolved whenever the function finishes completely.

Tip: The async / await syntax should look very familiar to readers with experience in C#, because it's basically identical.

The proposal essentially codifies support for the pattern we've already derived, into a syntactic mechanism: combining Promises with sync-looking flow control code. That's the best of both worlds combined, to effectively address practically all of the major concerns we outlined with callbacks.

The mere fact that such a ES7-ish proposal already exists and has early support and enthusiasm is a major vote of confidence in the future importance of this async pattern.

Promise Concurrency in Generators

So far, all we've demonstrated is a single-step async flow with Promises+generators. But real-world code will often have many async steps.

If you're not careful, the sync-looking style of generators may lull you into complacency with how you structure your async concurrency, leading to suboptimal performance patterns. So we want to spend a little time exploring the options.

Imagine a scenario where you need to fetch data from two different sources, then combine those responses to make a third request, and finally print out the last response. We explored a similar scenario with Promises in Chapter 3, but let's reconsider it in the context of generators.

Your first instinct might be something like:

function *foo() {
    var r1 = yield request( "http://some.url.1" );
    var r2 = yield request( "http://some.url.2" );

    var r3 = yield request(
        "http://some.url.3/?v=" + r1 + "," + r2
    );

    console.log( r3 );
}

// use previously defined `run(..)` utility
run( foo );

This code will work, but in the specifics of our scenario, it's not optimal. Can you spot why?

Because the r1 and r2 requests can -- and for performance reasons, should -- run concurrently, but in this code they will run sequentially; the "http://some.url.2" URL isn't Ajax fetched until after the "http://some.url.1" request is finished. These two requests are independent, so the better performance approach would likely be to have them run at the same time.

But how exactly would you do that with a generator and yield? We know that yield is only a single pause point in the code, so you can't really do two pauses at the same time.

The most natural and effective answer is to base the async flow on Promises, specifically on their capability to manage state in a time-independent fashion (see "Future Value" in Chapter 3).

The simplest approach:

function *foo() {
    // make both requests "in parallel"
    var p1 = request( "http://some.url.1" );
    var p2 = request( "http://some.url.2" );

    // wait until both promises resolve
    var r1 = yield p1;
    var r2 = yield p2;

    var r3 = yield request(
        "http://some.url.3/?v=" + r1 + "," + r2
    );

    console.log( r3 );
}

// use previously defined `run(..)` utility
run( foo );

Why is this different from the previous snippet? Look at where the yield is and is not. p1 and p2 are promises for Ajax requests made concurrently (aka "in parallel"). It doesn't matter which one finishes first, because promises will hold onto their resolved state for as long as necessary.

Then we use two subsequent yield statements to wait for and retrieve the resolutions from the promises (into r1 and r2, respectively). If p1 resolves first, the yield p1 resumes first then waits on the yield p2 to resume. If p2 resolves first, it will just patiently hold onto that resolution value until asked, but the yield p1 will hold on first, until p1 resolves.

Either way, both p1 and p2 will run concurrently, and both have to finish, in either order, before the r3 = yield request.. Ajax request will be made.

If that flow control processing model sounds familiar, it's basically the same as what we identified in Chapter 3 as the "gate" pattern, enabled by the Promise.all([ .. ]) utility. So, we could also express the flow control like this:

function *foo() {
    // make both requests "in parallel," and
    // wait until both promises resolve
    var results = yield Promise.all( [
        request( "http://some.url.1" ),
        request( "http://some.url.2" )
    ] );

    var r1 = results[0];
    var r2 = results[1];

    var r3 = yield request(
        "http://some.url.3/?v=" + r1 + "," + r2
    );

    console.log( r3 );
}

// use previously defined `run(..)` utility
run( foo );

Note: As we discussed in Chapter 3, we can even use ES6 destructuring assignment to simplify the var r1 = .. var r2 = .. assignments, with var [r1,r2] = results.

In other words, all of the concurrency capabilities of Promises are available to us in the generator+Promise approach. So in any place where you need more than sequential this-then-that async flow control steps, Promises are likely your best bet.

Promises, Hidden

As a word of stylistic caution, be careful about how much Promise logic you include inside your generators. The whole point of using generators for asynchrony in the way we've described is to create simple, sequential, sync-looking code, and to hide as much of the details of asynchrony away from that code as possible.

For example, this might be a cleaner approach:

// note: normal function, not generator
function bar(url1,url2) {
    return Promise.all( [
        request( url1 ),
        request( url2 )
    ] );
}

function *foo() {
    // hide the Promise-based concurrency details
    // inside `bar(..)`
    var results = yield bar(
        "http://some.url.1",
        "http://some.url.2"
    );

    var r1 = results[0];
    var r2 = results[1];

    var r3 = yield request(
        "http://some.url.3/?v=" + r1 + "," + r2
    );

    console.log( r3 );
}

// use previously defined `run(..)` utility
run( foo );

Inside *foo(), it's cleaner and clearer that all we're doing is just asking bar(..) to get us some results, and we'll yield-wait on that to happen. We don't have to care that under the covers a Promise.all([ .. ]) Promise composition will be used to make that happen.

We treat asynchrony, and indeed Promises, as an implementation detail.

Hiding your Promise logic inside a function that you merely call from your generator is especially useful if you're going to do a sophisticated series flow-control. For example:

function bar() {
    Promise.all( [
        baz( .. )
        .then( .. ),
        Promise.race( [ .. ] )
    ] )
    .then( .. )
}

That kind of logic is sometimes required, and if you dump it directly inside your generator(s), you've defeated most of the reason why you would want to use generators in the first place. We should intentionally abstract such details away from our generator code so that they don't clutter up the higher level task expression.

Beyond creating code that is both functional and performant, you should also strive to make code that is as reason-able and maintainable as possible.

Note: Abstraction is not always a healthy thing for programming -- many times it can increase complexity in exchange for terseness. But in this case, I believe it's much healthier for your generator+Promise async code than the alternatives. As with all such advice, though, pay attention to your specific situations and make proper decisions for you and your team.

Generator Delegation

In the previous section, we showed calling regular functions from inside a generator, and how that remains a useful technique for abstracting away implementation details (like async Promise flow). But the main drawback of using a normal function for this task is that it has to behave by the normal function rules, which means it cannot pause itself with yield like a generator can.

It may then occur to you that you might try to call one generator from another generator, using our run(..) helper, such as:

function *foo() {
    var r2 = yield request( "http://some.url.2" );
    var r3 = yield request( "http://some.url.3/?v=" + r2 );

    return r3;
}

function *bar() {
    var r1 = yield request( "http://some.url.1" );

    // "delegating" to `*foo()` via `run(..)`
    var r3 = yield run( foo );

    console.log( r3 );
}

run( bar );

We run *foo() inside of *bar() by using our run(..) utility again. We take advantage here of the fact that the run(..) we defined earlier returns a promise which is resolved when its generator is run to completion (or errors out), so if we yield out to a run(..) instance the promise from another run(..) call, it automatically pauses *bar() until *foo() finishes.

But there's an even better way to integrate calling *foo() into *bar(), and it's called yield-delegation. The special syntax for yield-delegation is: yield * __ (notice the extra *). Before we see it work in our previous example, let's look at a simpler scenario:

function *foo() {
    console.log( "`*foo()` starting" );
    yield 3;
    yield 4;
    console.log( "`*foo()` finished" );
}

function *bar() {
    yield 1;
    yield 2;
    yield *foo();    // `yield`-delegation!
    yield 5;
}

var it = bar();

it.next().value;    // 1
it.next().value;    // 2
it.next().value;    // `*foo()` starting
                    // 3
it.next().value;    // 4
it.next().value;    // `*foo()` finished
                    // 5

Note: Similar to a note earlier in the chapter where I explained why I prefer function *foo() .. instead of function* foo() .., I also prefer -- differing from most other documentation on the topic -- to say yield *foo() instead of yield* foo(). The placement of the * is purely stylistic and up to your best judgment. But I find the consistency of styling attractive.

How does the yield *foo() delegation work?

First, calling foo() creates an iterator exactly as we've already seen. Then, yield * delegates/transfers the iterator instance control (of the present *bar() generator) over to this other *foo() iterator.

So, the first two it.next() calls are controlling *bar(), but when we make the third it.next() call, now *foo() starts up, and now we're controlling *foo() instead of *bar(). That's why it's called delegation -- *bar() delegated its iteration control to *foo().

As soon as the it iterator control exhausts the entire *foo() iterator, it automatically returns to controlling *bar().

So now back to the previous example with the three sequential Ajax requests:

function *foo() {
    var r2 = yield request( "http://some.url.2" );
    var r3 = yield request( "http://some.url.3/?v=" + r2 );

    return r3;
}

function *bar() {
    var r1 = yield request( "http://some.url.1" );

    // "delegating" to `*foo()` via `yield*`
    var r3 = yield *foo();

    console.log( r3 );
}

run( bar );

The only difference between this snippet and the version used earlier is the use of yield *foo() instead of the previous yield run(foo).

Note: yield * yields iteration control, not generator control; when you invoke the *foo() generator, you're now yield-delegating to its iterator. But you can actually yield-delegate to any iterable; yield *[1,2,3] would consume the default iterator for the [1,2,3] array value.

Why Delegation?

The purpose of yield-delegation is mostly code organization, and in that way is symmetrical with normal function calling.

Imagine two modules that respectively provide methods foo() and bar(), where bar() calls foo(). The reason the two are separate is generally because the proper organization of code for the program calls for them to be in separate functions. For example, there may be cases where foo() is called standalone, and other places where bar() calls foo().

For all these exact same reasons, keeping generators separate aids in program readability, maintenance, and debuggability. In that respect, yield * is a syntactic shortcut for manually iterating over the steps of *foo() while inside of *bar().

Such manual approach would be especially complex if the steps in *foo() were asynchronous, which is why you'd probably need to use that run(..) utility to do it. And as we've shown, yield *foo() eliminates the need for a sub-instance of the run(..) utility (like run(foo)).

Delegating Messages

You may wonder how this yield-delegation works not just with iterator control but with the two-way message passing. Carefully follow the flow of messages in and out, through the yield-delegation:

function *foo() {
    console.log( "inside `*foo()`:", yield "B" );

    console.log( "inside `*foo()`:", yield "C" );

    return "D";
}

function *bar() {
    console.log( "inside `*bar()`:", yield "A" );

    // `yield`-delegation!
    console.log( "inside `*bar()`:", yield *foo() );

    console.log( "inside `*bar()`:", yield "E" );

    return "F";
}

var it = bar();

console.log( "outside:", it.next().value );
// outside: A

console.log( "outside:", it.next( 1 ).value );
// inside `*bar()`: 1
// outside: B

console.log( "outside:", it.next( 2 ).value );
// inside `*foo()`: 2
// outside: C

console.log( "outside:", it.next( 3 ).value );
// inside `*foo()`: 3
// inside `*bar()`: D
// outside: E

console.log( "outside:", it.next( 4 ).value );
// inside `*bar()`: 4
// outside: F

Pay particular attention to the processing steps after the it.next(3) call:

1. The 3 value is passed (through the yield-delegation in *bar()) into the waiting yield "C" expression inside of *foo().
2. *foo() then calls return "D", but this value doesn't get returned all the way back to the outside it.next(3) call.
3. Instead, the "D" value is sent as the result of the waiting yield *foo() expression inside of *bar() -- this yield-delegation expression has essentially been paused while all of *foo() was exhausted. So "D" ends up inside of *bar() for it to print out.
4. yield "E" is called inside of *bar(), and the "E" value is yielded to the outside as the result of the it.next(3) call.

From the perspective of the external iterator (it), it doesn't appear any differently between controlling the initial generator or a delegated one.

In fact, yield-delegation doesn't even have to be directed to another generator; it can just be directed to a non-generator, general iterable. For example:

function *bar() {
    console.log( "inside `*bar()`:", yield "A" );

    // `yield`-delegation to a non-generator!
    console.log( "inside `*bar()`:", yield *[ "B", "C", "D" ] );

    console.log( "inside `*bar()`:", yield "E" );

    return "F";
}

var it = bar();

console.log( "outside:", it.next().value );
// outside: A

console.log( "outside:", it.next( 1 ).value );
// inside `*bar()`: 1
// outside: B

console.log( "outside:", it.next( 2 ).value );
// outside: C

console.log( "outside:", it.next( 3 ).value );
// outside: D

console.log( "outside:", it.next( 4 ).value );
// inside `*bar()`: undefined
// outside: E

console.log( "outside:", it.next( 5 ).value );
// inside `*bar()`: 5
// outside: F

Notice the differences in where the messages were received/reported between this example and the one previous.

Most strikingly, the default array iterator doesn't care about any messages sent in via next(..) calls, so the values 2, 3, and 4 are essentially ignored. Also, because that iterator has no explicit return value (unlike the previously used *foo()), the yield * expression gets an undefined when it finishes.

Exceptions Delegated, Too!

In the same way that yield-delegation transparently passes messages through in both directions, errors/exceptions also pass in both directions:

function *foo() {
    try {
        yield "B";
    }
    catch (err) {
        console.log( "error caught inside `*foo()`:", err );
    }

    yield "C";

    throw "D";
}

function *bar() {
    yield "A";

    try {
        yield *foo();
    }
    catch (err) {
        console.log( "error caught inside `*bar()`:", err );
    }

    yield "E";

    yield *baz();

    // note: can't get here!
    yield "G";
}

function *baz() {
    throw "F";
}

var it = bar();

console.log( "outside:", it.next().value );
// outside: A

console.log( "outside:", it.next( 1 ).value );
// outside: B

console.log( "outside:", it.throw( 2 ).value );
// error caught inside `*foo()`: 2
// outside: C

console.log( "outside:", it.next( 3 ).value );
// error caught inside `*bar()`: D
// outside: E

try {
    console.log( "outside:", it.next( 4 ).value );
}
catch (err) {
    console.log( "error caught outside:", err );
}
// error caught outside: F

Some things to note from this snippet:

1. When we call it.throw(2), it sends the error message 2 into *bar(), which delegates that to *foo(), which then catches it and handles it gracefully. Then, the yield "C" sends "C" back out as the return value from the it.throw(2) call.
2. The "D" value that's next thrown from inside *foo() propagates out to *bar(), which catches it and handles it gracefully. Then the yield "E" sends "E" back out as the return value from the it.next(3) call.
3. Next, the exception thrown from *baz() isn't caught in *bar() -- though we did catch it outside -- so both *baz() and *bar() are set to a completed state. After this snippet, you would not be able to get the "G" value out with any subsequent next(..) call(s) -- they will just return undefined for value.

Delegating Asynchrony

Let's finally get back to our earlier yield-delegation example with the multiple sequential Ajax requests:

function *foo() {
    var r2 = yield request( "http://some.url.2" );
    var r3 = yield request( "http://some.url.3/?v=" + r2 );

    return r3;
}

function *bar() {
    var r1 = yield request( "http://some.url.1" );

    var r3 = yield *foo();

    console.log( r3 );
}

run( bar );

Instead of calling yield run(foo) inside of *bar(), we just call yield *foo().

In the previous version of this example, the Promise mechanism (controlled by run(..)) was used to transport the value from return r3 in *foo() to the local variable r3 inside *bar(). Now, that value is just returned back directly via the yield * mechanics.

Otherwise, the behavior is pretty much identical.

Delegating "Recursion"

Of course, yield-delegation can keep following as many delegation steps as you wire up. You could even use yield-delegation for async-capable generator "recursion" -- a generator yield-delegating to itself:

function *foo(val) {
    if (val > 1) {
        // generator recursion
        val = yield *foo( val - 1 );
    }

    return yield request( "http://some.url/?v=" + val );
}

function *bar() {
    var r1 = yield *foo( 3 );
    console.log( r1 );
}

run( bar );

Note: Our run(..) utility could have been called with run( foo, 3 ), because it supports additional parameters being passed along to the initialization of the generator. However, we used a parameter-free *bar() here to highlight the flexibility of yield *.

What processing steps follow from that code? Hang on, this is going to be quite intricate to describe in detail:

1. run(bar) starts up the *bar() generator.
2. foo(3) creates an iterator for *foo(..) and passes 3 as its val parameter.
3. Because 3 > 1, foo(2) creates another iterator and passes in 2 as its val parameter.
4. Because 2 > 1, foo(1) creates yet another iterator and passes in 1 as its val parameter.
5. 1 > 1 is false, so we next call request(..) with the 1 value, and get a promise back for that first Ajax call.
6. That promise is yielded out, which comes back to the *foo(2) generator instance.
7. The yield * passes that promise back out to the *foo(3) generator instance. Another yield * passes the promise out to the *bar() generator instance. And yet again another yield * passes the promise out to the run(..) utility, which will wait on that promise (for the first Ajax request) to proceed.
8. When the promise resolves, its fulfillment message is sent to resume *bar(), which passes through the yield * into the *foo(3) instance, which then passes through the yield * to the *foo(2) generator instance, which then passes through the yield * to the normal yield that's waiting in the *foo(3) generator instance.
9. That first call's Ajax response is now immediately returned from the *foo(3) generator instance, which sends that value back as the result of the yield * expression in the *foo(2) instance, and assigned to its local val variable.
10. Inside *foo(2), a second Ajax request is made with request(..), whose promise is yielded back to the *foo(1) instance, and then yield * propagates all the way out to run(..) (step 7 again). When the promise resolves, the second Ajax response propagates all the way back into the *foo(2) generator instance, and is assigned to its local val variable.
11. Finally, the third Ajax request is made with request(..), its promise goes out to run(..), and then its resolution value comes all the way back, which is then returned so that it comes back to the waiting yield * expression in *bar().

Phew! A lot of crazy mental juggling, huh? You might want to read through that a few more times, and then go grab a snack to clear your head!

Generator Concurrency

As we discussed in both Chapter 1 and earlier in this chapter, two simultaneously running "processes" can cooperatively interleave their operations, and many times this can yield (pun intended) very powerful asynchrony expressions.

Frankly, our earlier examples of concurrency interleaving of multiple generators showed how to make it really confusing. But we hinted that there's places where this capability is quite useful.

Recall a scenario we looked at in Chapter 1, where two different simultaneous Ajax response handlers needed to coordinate with each other to make sure that the data communication was not a race condition. We slotted the responses into the res array like this:

function response(data) {
    if (data.url == "http://some.url.1") {
        res[0] = data;
    }
    else if (data.url == "http://some.url.2") {
        res[1] = data;
    }
}

But how can we use multiple generators concurrently for this scenario?

// `request(..)` is a Promise-aware Ajax utility

var res = [];

function *reqData(url) {
    res.push(
        yield request( url )
    );
}

Note: We're going to use two instances of the *reqData(..) generator here, but there's no difference to running a single instance of two different generators; both approaches are reasoned about identically. We'll see two different generators coordinating in just a bit.

Instead of having to manually sort out res[0] and res[1] assignments, we'll use coordinated ordering so that res.push(..) properly slots the values in the expected and predictable order. The expressed logic thus should feel a bit cleaner.

But how will we actually orchestrate this interaction? First, let's just do it manually, with Promises:

var it1 = reqData( "http://some.url.1" );
var it2 = reqData( "http://some.url.2" );

var p1 = it1.next();
var p2 = it2.next();

p1
.then( function(data){
    it1.next( data );
    return p2;
} )
.then( function(data){
    it2.next( data );
} );

*reqData(..)'s two instances are both started to make their Ajax requests, then paused with yield. Then we choose to resume the first instance when p1 resolves, and then p2's resolution will restart the second instance. In this way, we use Promise orchestration to ensure that res[0] will have the first response and res[1] will have the second response.

But frankly, this is awfully manual, and it doesn't really let the generators orchestrate themselves, which is where the true power can lie. Let's try it a different way:

// `request(..)` is a Promise-aware Ajax utility

var res = [];

function *reqData(url) {
    var data = yield request( url );

    // transfer control
    yield;

    res.push( data );
}

var it1 = reqData( "http://some.url.1" );
var it2 = reqData( "http://some.url.2" );

var p1 = it1.next();
var p2 = it2.next();

p1.then( function(data){
    it1.next( data );
} );

p2.then( function(data){
    it2.next( data );
} );

Promise.all( [p1,p2] )
.then( function(){
    it1.next();
    it2.next();
} );

OK, this is a bit better (though still manual!), because now the two instances of *reqData(..) run truly concurrently, and (at least for the first part) independently.

In the previous snippet, the second instance was not given its data until after the first instance was totally finished. But here, both instances receive their data as soon as their respective responses come back, and then each instance does another yield for control transfer purposes. We then choose what order to resume them in the Promise.all([ .. ]) handler.

What may not be as obvious is that this approach hints at an easier form for a reusable utility, because of the symmetry. We can do even better. Let's imagine using a utility called runAll(..):

// `request(..)` is a Promise-aware Ajax utility

var res = [];

runAll(
    function*(){
        var p1 = request( "http://some.url.1" );

        // transfer control
        yield;

        res.push( yield p1 );
    },
    function*(){
        var p2 = request( "http://some.url.2" );

        // transfer control
        yield;

        res.push( yield p2 );
    }
);

Note: We're not including a code listing for runAll(..) as it is not only long enough to bog down the text, but is an extension of the logic we've already implemented in run(..) earlier. So, as a good supplementary exercise for the reader, try your hand at evolving the code from run(..) to work like the imagined runAll(..). Also, my asynquence library provides a previously mentioned runner(..) utility with this kind of capability already built in, and will be discussed in Appendix A of this book.

Here's how the processing inside runAll(..) would operate:

1. The first generator gets a promise for the first Ajax response from "http://some.url.1", then yields control back to the runAll(..) utility.
2. The second generator runs and does the same for "http://some.url.2", yielding control back to the runAll(..) utility.
3. The first generator resumes, and then yields out its promise p1. The runAll(..) utility does the same in this case as our previous run(..), in that it waits on that promise to resolve, then resumes the same generator (no control transfer!). When p1 resolves, runAll(..) resumes the first generator again with that resolution value, and then res[0] is given its value. When the first generator then finishes, that's an implicit transfer of control.
4. The second generator resumes, yields out its promise p2, and waits for it to resolve. Once it does, runAll(..) resumes the second generator with that value, and res[1] is set.

In this running example, we use an outer variable called res to store the results of the two different Ajax responses -- that's our concurrency coordination making that possible.

But it might be quite helpful to further extend runAll(..) to provide an inner variable space for the multiple generator instances to share, such as an empty object we'll call data below. Also, it could take non-Promise values that are yielded and hand them off to the next generator.

Consider:

// `request(..)` is a Promise-aware Ajax utility

runAll(
    function*(data){
        data.res = [];

        // transfer control (and message pass)
        var url1 = yield "http://some.url.2";

        var p1 = request( url1 ); // "http://some.url.1"

        // transfer control
        yield;

        data.res.push( yield p1 );
    },
    function*(data){
        // transfer control (and message pass)
        var url2 = yield "http://some.url.1";

        var p2 = request( url2 ); // "http://some.url.2"

        // transfer control
        yield;

        data.res.push( yield p2 );
    }
);

In this formulation, the two generators are not just coordinating control transfer, but actually communicating with each other, both through data.res and the yielded messages that trade url1 and url2 values. That's incredibly powerful!

Such realization also serves as a conceptual base for a more sophisticated asynchrony technique called CSP (Communicating Sequential Processes), which we will cover in Appendix B of this book.

Thunks

So far, we've made the assumption that yielding a Promise from a generator -- and having that Promise resume the generator via a helper utility like run(..) -- was the best possible way to manage asynchrony with generators. To be clear, it is.

But we skipped over another pattern that has some mildly widespread adoption, so in the interest of completeness we'll take a brief look at it.

In general computer science, there's an old pre-JS concept called a "thunk." Without getting bogged down in the historical nature, a narrow expression of a thunk in JS is a function that -- without any parameters -- is wired to call another function.

In other words, you wrap a function definition around function call -- with any parameters it needs -- to defer the execution of that call, and that wrapping function is a thunk. When you later execute the thunk, you end up calling the original function.

For example:

function foo(x,y) {
    return x + y;
}

function fooThunk() {
    return foo( 3, 4 );
}

// later

console.log( fooThunk() );    // 7

So, a synchronous thunk is pretty straightforward. But what about an async thunk? We can essentially extend the narrow thunk definition to include it receiving a callback.

Consider:

function foo(x,y,cb) {
    setTimeout( function(){
        cb( x + y );
    }, 1000 );
}

function fooThunk(cb) {
    foo( 3, 4, cb );
}

// later

fooThunk( function(sum){
    console.log( sum );        // 7
} );

As you can see, fooThunk(..) only expects a cb(..) parameter, as it already has values 3 and 4 (for x and y, respectively) pre-specified and ready to pass to foo(..). A thunk is just waiting around patiently for the last piece it needs to do its job: the callback.

You don't want to make thunks manually, though. So, let's invent a utility that does this wrapping for us.

Consider:

function thunkify(fn) {
    var args = [].slice.call( arguments, 1 );
    return function(cb) {
        args.push( cb );
        return fn.apply( null, args );
    };
}

var fooThunk = thunkify( foo, 3, 4 );

// later

fooThunk( function(sum) {
    console.log( sum );        // 7
} );

Tip: Here we assume that the original (foo(..)) function signature expects its callback in the last position, with any other parameters coming before it. This is a pretty ubiquitous "standard" for async JS function standards. You might call it "callback-last style." If for some reason you had a need to handle "callback-first style" signatures, you would just make a utility that used args.unshift(..) instead of args.push(..).

The preceding formulation of thunkify(..) takes both the foo(..) function reference, and any parameters it needs, and returns back the thunk itself (fooThunk(..)). However, that's not the typical approach you'll find to thunks in JS.

Instead of thunkify(..) making the thunk itself, typically -- if not perplexingly -- the thunkify(..) utility would produce a function that produces thunks.

Uhhhh... yeah.

Consider:

function thunkify(fn) {
    return function() {
        var args = [].slice.call( arguments );
        return function(cb) {
            args.push( cb );
            return fn.apply( null, args );
        };
    };
}

The main difference here is the extra return function() { .. } layer. Here's how its usage differs:

var whatIsThis = thunkify( foo );

var fooThunk = whatIsThis( 3, 4 );

// later

fooThunk( function(sum) {
    console.log( sum );        // 7
} );

Obviously, the big question this snippet implies is what is whatIsThis properly called? It's not the thunk, it's the thing that will produce thunks from foo(..) calls. It's kind of like a "factory" for "thunks." There doesn't seem to be any kind of standard agreement for naming such a thing.

So, my proposal is "thunkory" ("thunk" + "factory"). So, thunkify(..) produces a thunkory, and a thunkory produces thunks. That reasoning is symmetric to my proposal for "promisory" in Chapter 3:

var fooThunkory = thunkify( foo );

var fooThunk1 = fooThunkory( 3, 4 );
var fooThunk2 = fooThunkory( 5, 6 );

// later

fooThunk1( function(sum) {
    console.log( sum );        // 7
} );

fooThunk2( function(sum) {
    console.log( sum );        // 11
} );

Note: The running foo(..) example expects a style of callback that's not "error-first style." Of course, "error-first style" is much more common. If foo(..) had some sort of legitimate error-producing expectation, we could change it to expect and use an error-first callback. None of the subsequent thunkify(..) machinery cares what style of callback is assumed. The only difference in usage would be fooThunk1(function(err,sum){...

Exposing the thunkory method -- instead of how the earlier thunkify(..) hides this intermediary step -- may seem like unnecessary complication. But in general, it's quite useful to make thunkories at the beginning of your program to wrap existing API methods, and then be able to pass around and call those thunkories when you need thunks. The two distinct steps preserve a cleaner separation of capability.

To illustrate:

// cleaner:
var fooThunkory = thunkify( foo );

var fooThunk1 = fooThunkory( 3, 4 );
var fooThunk2 = fooThunkory( 5, 6 );

// instead of:
var fooThunk1 = thunkify( foo, 3, 4 );
var fooThunk2 = thunkify( foo, 5, 6 );

Regardless of whether you like to deal with the thunkories explicitly or not, the usage of thunks fooThunk1(..) and fooThunk2(..) remains the same.

s/promise/thunk/

So what's all this thunk stuff have to do with generators?

Comparing thunks to promises generally: they're not directly interchangable as they're not equivalent in behavior. Promises are vastly more capable and trustable than bare thunks.

But in another sense, they both can be seen as a request for a value, which may be async in its answering.

Recall from Chapter 3 we defined a utility for promisifying a function, which we called Promise.wrap(..) -- we could have called it promisify(..), too! This Promise-wrapping utility doesn't produce Promises; it produces promisories that in turn produce Promises. This is completely symmetric to the thunkories and thunks presently being discussed.

To illustrate the symmetry, let's first alter the running foo(..) example from earlier to assume an "error-first style" callback:

function foo(x,y,cb) {
    setTimeout( function(){
        // assume `cb(..)` as "error-first style"
        cb( null, x + y );
    }, 1000 );
}

Now, we'll compare using thunkify(..) and promisify(..) (aka Promise.wrap(..) from Chapter 3):

// symmetrical: constructing the question asker
var fooThunkory = thunkify( foo );
var fooPromisory = promisify( foo );

// symmetrical: asking the question
var fooThunk = fooThunkory( 3, 4 );
var fooPromise = fooPromisory( 3, 4 );

// get the thunk answer
fooThunk( function(err,sum){
    if (err) {
        console.error( err );
    }
    else {
        console.log( sum );        // 7
    }
} );

// get the promise answer
fooPromise
.then(
    function(sum){
        console.log( sum );        // 7
    },
    function(err){
        console.error( err );
    }
);

Both the thunkory and the promisory are essentially asking a question (for a value), and respectively the thunk fooThunk and promise fooPromise represent the future answers to that question. Presented in that light, the symmetry is clear.

With that perspective in mind, we can see that generators which yield Promises for asynchrony could instead yield thunks for asynchrony. All we'd need is a smarter run(..) utility (like from before) that can not only look for and wire up to a yielded Promise but also to provide a callback to a yielded thunk.

Consider:

function *foo() {
    var val = yield request( "http://some.url.1" );
    console.log( val );
}

run( foo );

In this example, request(..) could either be a promisory that returns a promise, or a thunkory that returns a thunk. From the perspective of what's going on inside the generator code logic, we don't care about that implementation detail, which is quite powerful!

So, request(..) could be either:

// promisory `request(..)` (see Chapter 3)
var request = Promise.wrap( ajax );

// vs.

// thunkory `request(..)`
var request = thunkify( ajax );

Finally, as a thunk-aware patch to our earlier run(..) utility, we would need logic like this:

// ..
// did we receive a thunk back?
else if (typeof next.value == "function") {
    return new Promise( function(resolve,reject){
        // call the thunk with an error-first callback
        next.value( function(err,msg) {
            if (err) {
                reject( err );
            }
            else {
                resolve( msg );
            }
        } );
    } )
    .then(
        handleNext,
        function handleErr(err) {
            return Promise.resolve(
                it.throw( err )
            )
            .then( handleResult );
        }
    );
}

Now, our generators can either call promisories to yield Promises, or call thunkories to yield thunks, and in either case, run(..) would handle that value and use it to wait for the completion to resume the generator.

Symmetry wise, these two approaches look identical. However, we should point out that's true only from the perspective of Promises or thunks representing the future value continuation of a generator.

From the larger perspective, thunks do not in and of themselves have hardly any of the trustability or composability guarantees that Promises are designed with. Using a thunk as a stand-in for a Promise in this particular generator asynchrony pattern is workable but should be seen as less than ideal when compared to all the benefits that Promises offer (see Chapter 3).

If you have the option, prefer yield pr rather than yield th. But there's nothing wrong with having a run(..) utility which can handle both value types.

Note: The runner(..) utility in my asynquence library, which will be discussed in Appendix A, handles yields of Promises, thunks and asynquence sequences.

Pre-ES6 Generators

You're hopefully convinced now that generators are a very important addition to the async programming toolbox. But it's a new syntax in ES6, which means you can't just polyfill generators like you can Promises (which are just a new API). So what can we do to bring generators to our browser JS if we don't have the luxury of ignoring pre-ES6 browsers?

For all new syntax extensions in ES6, there are tools -- the most common term for them is transpilers, for trans-compilers -- which can take your ES6 syntax and transform it into equivalent (but obviously uglier!) pre-ES6 code. So, generators can be transpiled into code that will have the same behavior but work in ES5 and below.

But how? The "magic" of yield doesn't obviously sound like code that's easy to transpile. We actually hinted at a solution in our earlier discussion of closure-based iterators.

Manual Transformation

Before we discuss the transpilers, let's derive how manual transpilation would work in the case of generators. This isn't just an academic exercise, because doing so will actually help further reinforce how they work.

Consider:

// `request(..)` is a Promise-aware Ajax utility

function *foo(url) {
    try {
        console.log( "requesting:", url );
        var val = yield request( url );
        console.log( val );
    }
    catch (err) {
        console.log( "Oops:", err );
        return false;
    }
}

var it = foo( "http://some.url.1" );

The first thing to observe is that we'll still need a normal foo() function that can be called, and it will still need to return an iterator. So, let's sketch out the non-generator transformation:

function foo(url) {

    // ..

    // make and return an iterator
    return {
        next: function(v) {
            // ..
        },
        throw: function(e) {
            // ..
        }
    };
}

var it = foo( "http://some.url.1" );

The next thing to observe is that a generator does its "magic" by suspending its scope/state, but we can emulate that with function closure (see the Scope & Closures title of this series). To understand how to write such code, we'll first annotate different parts of our generator with state values:

// `request(..)` is a Promise-aware Ajax utility

function *foo(url) {
    // STATE *1*

    try {
        console.log( "requesting:", url );
        var TMP1 = request( url );

        // STATE *2*
        var val = yield TMP1;
        console.log( val );
    }
    catch (err) {
        // STATE *3*
        console.log( "Oops:", err );
        return false;
    }
}

Note: For more accurate illustration, we split up the val = yield request.. statement into two parts, using the temporary TMP1 variable. request(..) happens in state *1*, and the assignment of its completion value to val happens in state *2*. We'll get rid of that intermediate TMP1 when we convert the code to its non-generator equivalent.

In other words, *1* is the beginning state, *2* is the state if the request(..) succeeds, and *3* is the state if the request(..) fails. You can probably imagine how any extra yield steps would just be encoded as extra states.

Back to our transpiled generator, let's define a variable state in the closure we can use to keep track of the state:

function foo(url) {
    // manage generator state
    var state;

    // ..
}

Now, let's define an inner function called process(..) inside the closure which handles each state, using a switch statement:

// `request(..)` is a Promise-aware Ajax utility

function foo(url) {
    // manage generator state
    var state;

    // generator-wide variable declarations
    var val;

    function process(v) {
        switch (state) {
            case 1:
                console.log( "requesting:", url );
                return request( url );
            case 2:
                val = v;
                console.log( val );
                return;
            case 3:
                var err = v;
                console.log( "Oops:", err );
                return false;
        }
    }

    // ..
}

Each state in our generator is represented by its own case in the switch statement. process(..) will be called each time we need to process a new state. We'll come back to how that works in just a moment.

For any generator-wide variable declarations (val), we move those to a var declaration outside of process(..) so they can survive multiple calls to process(..). But the "block scoped" err variable is only needed for the *3* state, so we leave it in place.

In state *1*, instead of yield request(..), we did return request(..). In terminal state *2*, there was no explicit return, so we just do a return; which is the same as return undefined. In terminal state *3*, there was a return false, so we preserve that.

Now we need to define the code in the iterator functions so they call process(..) appropriately:

function foo(url) {
    // manage generator state
    var state;

    // generator-wide variable declarations
    var val;

    function process(v) {
        switch (state) {
            case 1:
                console.log( "requesting:", url );
                return request( url );
            case 2:
                val = v;
                console.log( val );
                return;
            case 3:
                var err = v;
                console.log( "Oops:", err );
                return false;
        }
    }

    // make and return an iterator
    return {
        next: function(v) {
            // initial state
            if (!state) {
                state = 1;
                return {
                    done: false,
                    value: process()
                };
            }
            // yield resumed successfully
            else if (state == 1) {
                state = 2;
                return {
                    done: true,
                    value: process( v )
                };
            }
            // generator already completed
            else {
                return {
                    done: true,
                    value: undefined
                };
            }
        },
        "throw": function(e) {
            // the only explicit error handling is in
            // state *1*
            if (state == 1) {
                state = 3;
                return {
                    done: true,
                    value: process( e )
                };
            }
            // otherwise, an error won't be handled,
            // so just throw it right back out
            else {
                throw e;
            }
        }
    };
}

How does this code work?

1. The first call to the iterator's next() call would move the generator from the unitialized state to state 1, and then call process() to handle that state. The return value from request(..), which is the promise for the Ajax response, is returned back as the value property from the next() call.
2. If the Ajax request succeeds, the second call to next(..) should send in the Ajax response value, which moves our state to 2. process(..) is again called (this time with the passed in Ajax response value), and the value property returned from next(..) will be undefined.
3. However, if the Ajax request fails, throw(..) should be called with the error, which would move the state from 1 to 3 (instead of 2). Again process(..) is called, this time with the error value. That case returns false, which is set as the value property returned from the throw(..) call.

From the outside -- that is, interacting only with the iterator -- this foo(..) normal function works pretty much the same as the *foo(..) generator would have worked. So we've effectively "transpiled" our ES6 generator to pre-ES6 compatibility!

We could then manually instantiate our generator and control its iterator -- calling var it = foo("..") and it.next(..) and such -- or better, we could pass it to our previously defined run(..) utility as run(foo,"..").

Automatic Transpilation

The preceding exercise of manually deriving a transformation of our ES6 generator to pre-ES6 equivalent teaches us how generators work conceptually. But that transformation was really intricate and very non-portable to other generators in our code. It would be quite impractical to do this work by hand, and would completely obviate all the benefit of generators.

But luckily, several tools already exist that can automatically convert ES6 generators to things like what we derived in the previous section. Not only do they do the heavy lifting work for us, but they also handle several complications that we glossed over.

One such tool is regenerator (https://facebook.github.io/regenerator/), from the smart folks at Facebook.

If we use regenerator to transpile our previous generator, here's the code produced (at the time of this writing):

// `request(..)` is a Promise-aware Ajax utility

var foo = regeneratorRuntime.mark(function foo(url) {
    var val;

    return regeneratorRuntime.wrap(function foo$(context$1$0) {
        while (1) switch (context$1$0.prev = context$1$0.next) {
        case 0:
            context$1$0.prev = 0;
            console.log( "requesting:", url );
            context$1$0.next = 4;
            return request( url );
        case 4:
            val = context$1$0.sent;
            console.log( val );
            context$1$0.next = 12;
            break;
        case 8:
            context$1$0.prev = 8;
            context$1$0.t0 = context$1$0.catch(0);
            console.log("Oops:", context$1$0.t0);
            return context$1$0.abrupt("return", false);
        case 12:
        case "end":
            return context$1$0.stop();
        }
    }, foo, this, [[0, 8]]);
});

There's some obvious similarities here to our manual derivation, such as the switch / case statements, and we even see val pulled out of the closure just as we did.

Of course, one trade-off is that regenerator's transpilation requires a helper library regeneratorRuntime that holds all the reusable logic for managing a general generator / iterator. A lot of that boilerplate looks different than our version, but even then, the concepts can be seen, like with context$1$0.next = 4 keeping track of the next state for the generator.

The main takeaway is that generators are not restricted to only being useful in ES6+ environments. Once you understand the concepts, you can employ them throughout your code, and use tools to transform the code to be compatible with older environments.

This is more work than just using a Promise API polyfill for pre-ES6 Promises, but the effort is totally worth it, because generators are so much better at expressing async flow control in a reason-able, sensible, synchronous-looking, sequential fashion.

Once you get hooked on generators, you'll never want to go back to the hell of async spaghetti callbacks!

Review

Generators are a new ES6 function type that does not run-to-completion like normal functions. Instead, the generator can be paused in mid-completion (entirely preserving its state), and it can later be resumed from where it left off.

This pause/resume interchange is cooperative rather than preemptive, which means that the generator has the sole capability to pause itself, using the yield keyword, and yet the iterator that controls the generator has the sole capability (via next(..)) to resume the generator.

The yield / next(..) duality is not just a control mechanism, it's actually a two-way message passing mechanism. A yield .. expression essentially pauses waiting for a value, and the next next(..) call passes a value (or implicit undefined) back to that paused yield expression.

The key benefit of generators related to async flow control is that the code inside a generator expresses a sequence of steps for the task in a naturally sync/sequential fashion. The trick is that we essentially hide potential asynchrony behind the yield keyword -- moving the asynchrony to the code where the generator's iterator is controlled.

In other words, generators preserve a sequential, synchronous, blocking code pattern for async code, which lets our brains reason about the code much more naturally, addressing one of the two key drawbacks of callback-based async.