Iterators and generators

Iterators

The idea of an iterator is to loop through an array / collection of data. Typically this is done with for and while loops.

Generators

Anything that has the capability of creating a sequence is a generator.

function positiveInts(n) {
  var i = 1;
  var max = n < 1 || typeof n !== "number" ? 1 : n;
  return {
    next: function () {
      if (i > max) return { value: undefined, done: true };
      return { value: i++, done: false };
    },
  };
}

var seq = positiveInts(3);

seq.next(); // {value: 1, done: false}
seq.next(); // {value: 2, done: false}
seq.next(); // {value: 3, done: false}
seq.next(); // {value: undefined, done: true}

Generator function

An asterisk ** is used to define a generator function.

function* myGenerator() {
  // do things
}

The yield keyword is used to define a value in the iterable sequence.

function* sequence() {
  yield 1;
  yield 3;
  yield 5;
}

var seq = sequence();

seq.next(); // {value: 1, done: false}
seq.next(); // {value: 3, done: false}
seq.next(); // {value: 5, done: false}
seq.next(); // {value: undefined, done: true}

The code var seq = sequence(); does not execute the logic inside the sequence() function but rather just gets an instance of the generator.

When the call next() is made on the sequence instance each of the yield values is returned one by one.

Each time the sequence is called it goes through the code until a yield is reached and then pauses the execution. The current position is internally retained in the generator function. On subsequent calls it moves again from the last position until the next yield.

• next() resumes execution within the generator.
• yield pauses execution within the generator.