Generator Functions in JavaScript with ES6 / ES2015

Published on November 7, 2016

JavaScript

By Alligator.io

Developer and author at DigitalOcean.

Generator Functions in JavaScript with ES6 / ES2015

While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.

With generator functions in ES2015 (ES6), we’re now able to define functions that can return multiple values with the help of the yield keyword. Here’s an example of a simple generator function:

function* someGenerator(){
  yield 'Cats';
  yield 'Dogs';
  yield 'Birds';
}

Notice the use of the * character next to the function keyword, to indicate that it’s a generator function.

Now let’s create an instance of the generator and extract the values from it by calling next() on the generator and getting value:

const gen = someGenerator();

console.log(gen.next().value); // Cats
console.log(gen.next().value); // Dogs
console.log(gen.next().value); // Birds
console.log(gen.next().value); // Undefined

next() returns an object with the value and a boolean called done that returns true if the generator is out of values:

const gen2 = someGenerator();

console.log(gen2.next().done); // false
console.log(gen2.next().done); // false
console.log(gen2.next().done); // false
console.log(gen2.next().done); // true

ID Generator

You can use an infinite while in a generator function to create a function that will generate a new ID every time next() is called on it. In this case the infinite loop is not dangerous because it will only execute once with calls to next():

function* idGen(){
  let i = 0;
  while (true) {
    yield ++i;
  }
}

const myIdGen = idGen();

console.log(myIdGen.next().value); // 1
console.log(myIdGen.next().value); // 2
console.log(myIdGen.next().value); // 3
console.log(myIdGen.next().value); // 4