Iterators

In JavaScript an iterator is an object that provides a next() method which returns the next item in the sequence.

String, Array, TypedArray, Map and Set are all built-in iterables, because the prototype objects of them all have a Symbol.iterator method.

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let myArray = [1,2];
let iterator = myArray[Symbol.iterator]();
console.log(iterator.next()); // { done: false, value: 1 }
console.log(iterator.next()); // { done: false, value: 2 }
console.log(iterator.next()); // { done: true, value: undefined }

Generators

A generator is a special type of function that works as a factory for iterators. A function becomes a generator if it contains one or more yield expressions and if it uses the function* syntax.

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function* generator() {
  yield 1;
  yield 2;
}
let iterator = generator();
console.log(iterator.next()); // { done: false, value: 1 }
console.log(iterator.next()); // { done: false, value: 2 }
console.log(iterator.next()); // { done: true, value: undefined }
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function* generator() {
  yield 1;
  yield* anotherGenerator();
  yield 3;
}
function* anotherGenerator() {
  yield 2;
}
let iterator = generator();
console.log(iterator.next()); // { done: false, value: 1 }
console.log(iterator.next()); // { done: false, value: 2 }
console.log(iterator.next()); // { done: false, value: 3 }
console.log(iterator.next()); // { done: true, value: undefined }

Örnek Kullanım

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function request(url) {
  return new Promise(function(resolve, reject) {
    makeAjaxCall(url, function(err, text) {
      if (err) reject(err);
      else resolve(text); 
    });
  });
}
function* generator(url) {
  yield request(url1);
  yield request(url2); // This can be run after promise from url1 resolved or rejected
}