Generator Functions in JavaScript with ES6 / ES2015


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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
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