What is a Generator Function?

A generator function is a different, special function.

Differentiating Syntax

Because it’s different, a generator function has a syntax to mark it as such:

function* myGenerator() {}

Note the * after function. This is the function declaration difference. The * can be applied to arrow functions ()* => {} or object methods class { myGenerator()* { } }.

Starts Paused

When you call a generator function, it doesn’t actually execute the body of the function. Rather, it “starts” paused. Instead of invoking the function, returing a return value, it returns a generator object, also called an iterator. An iterator is a special object that allows you to iterate through a collection.

If I call this generator function, the error will not be thrown:

function* generator() {
  throw new Error()
}

const iterator = generator()

We haven’t actually executed the first line of the function yet.

Can Advance Programmatically

The returned iterator conforms to an iterator protocol. This means that the object will have a special property, next, which is a function that, when called, advances the iterator.

function* generator() {
  throw new Error()
}

const iterator = generator()
iterator.next()

Since we’re calling next, we’re running the function body, and this example will throw an error. The next() function is meant to be called multiple times if necessary.

Can Pause Again

Inside of a generator function, you can use the keyword yield. It’s a little like await in that it defines a “pause point”. Said another way, it defines an element in the collection of things that the iterator is iterating over.

Thus every time you call iterator.next() you advance to the first yield, then all the yields thereafter. In this example, it will take 3 calls to next() to iterator through the entire iterator collection:

function* generator() { 
  yield 'fan'
  yield 'tast'
  yield 'ic'
}
const iterator = generator()

Can Return Multiple Values

The yields are useful for more than pausing. Each can return a value. The 3 calls to the iterator in the example above would yield the following:

iterator.next().value // 'fan'
iterator.next().value // 'tast'
iterator.next().value // 'ic'

Note the accessing of .value on the object that next() returns in order to access the yielded value.

Can Know When It’s Done

How do you know when to stop calling next()? After all, the generator in the example above has finite values to yield. We could check to see if next().value is undefined and then stop iterating. But what if undefined is a valid, expected value to be returned from this generator. For something much more sure, the object returned by next() also includes the property boolean done, as a sibling to value, which, if true, signifies the end of the iterable collection of yields.

To clarify the picture a bit, calls to next() return an object with { value, done }. This is an oddly long, possibly-ambiguous term, for this object because it is a rather specialized object. But it’s the best term I’ve been able to find.

Can Resume with Client Values

When we call next(), we’re resuming the generator, letting it run until it reaches the next yield. We can even provide our own values into the generator function. This is much like function parameters, but we’re able to provide them for every yield. We can effectively tell the generator what the value to be yielded is. To do this we provide an argument to next().

function* generator() {
  const result = yield 'simon'
  console.log(result)
}
const iterator = generator()
iterator.next('says')

Whatever we provide as the argument to next() will be the value that is yielded. The above example will print 'says' to the console.

Can Throw Errors

We can also throw errors within a generator function. We get this opportunity with each yield. In addition to next(), an iterator returned by a generator also has a throw() function. We can even provide our own error to the throw() function as an argument, similar to providing values to next(). So if we wanted to go into a catch block, we might write:

function* generator() {
  try {
    const result = yield 'trip wire'
  } catch (err) {
    console.log(err)
  }
}
const iterator = generator()
iterator.throw(new Error('boom!'))

Calling throw() will cause this example to print 'boom!' to the console.

Generator Function Applications

Now we know of a fairly interesting type of function: the generator. What can we use generators for? It starts paused, can pause again, can return multiple values, and we can feed it values in the middle of the function body. I’m interested to hear of the applications you’ve found. One of the most interesting I’ve found is in the area of effects as data.

An example library that is using generators for this purpose is redux-saga. The application of generators works well there because a generator function describing work to be done can have multiple yields on each step of work. The function can pause and exit to execute that work elsewhere. Then work as it is described in the generator can resume again until it’s done. There are more interesting points on effects as data. But for now know that specific application is compelling and makes good use of generators.

What other questions about generators do you have? What do you use them for?