Is there any "python's Generator" equivalent in JavaScript?

PS: Python's Generator is very memory efficient when we need to do one time iterate through a big array, hash...
"Generators are iterables, but you can only read them once. It's because they do not store all the values in memory, they generate the values on the fly"

(Python's Generator explained in this thread: What does the "yield" keyword do in Python? )

Is there any "python's Generator" equivalent in JavaScript?

PS: Python's Generator is very memory efficient when we need to do one time iterate through a big array, hash...
"Generators are iterables, but you can only read them once. It's because they do not store all the values in memory, they generate the values on the fly"

(Python's Generator explained in this thread: What does the "yield" keyword do in Python? )

• javascript
• iteration

4 Answers 4

Not in a standard way. Some browsers already implement python-style generators, but they require extensions that need to be explicitly activated. There are proposals to add generators to a next version of the ECMAScript spec, but I wouldn't see that being useable very soon.

So far, the best you can do is to use the old school external iterator pattern. It is just as powerful, although it is a pain to write in more plicated cases.

It's not super practical, but you can achieve the same basic effect like this:

function make_generator(start, end) {
  var i=start;
  return function() {
    if (i<end) {
      output = i;
      i += 1;
      return output;
    }
    else {
      return null;
    }
  }
}
var out = document.getElementById('out');
var generator = make_generator(1,10);
var g = generator();
while (g) {
  if (out.innerHTML)
    out.innerHTML = out.innerHTML + '<br>' + g;
  else
    out.innerHTML = g;
  g = generator();
}

You can use the yield keyword. You can read more about it here (MDN).

The yield keyword is used to pause and resume a generator function

function* range(n) {
    for (let i = 0; i < n; i++) {
        yield i;
    }
}

const generator = range(10);

console.log(generator.next());  // { value: 0, done: false }
console.log(generator.next());  // { value: 1, done: false }
...
console.log(generator.next());  // { value: 9, done: false }
console.log(generator.next());  // { value: undefined, done: true }

Notice the * after the function keyword.

It's widely supported now: caniuse, MDN

TypeScript notation:

function* range(n: number): IterableIterator<number> {
    for (let i = 0; i < n; i++) {
        yield i;
    }
}

In JavaScript 1.7:

function rangeGen(n) {
    for (let i = 0; i < n; i++)
        yield i;
}