Iterators in Javascript

Last updated Feb 8, 2023 Edit Source

• Iterators
• Javascript
• Lesson
• Class

Iterators in JavaScript are objects that define a sequence of values and a next() method to access them. The next() method returns an object with two properties: value (the current item in the sequence) and done (a boolean indicating whether the iteration has finished).

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let myArray = [1, 2, 3];
let it = myArray[Symbol.iterator]();

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

In the example above, myArray[Symbol.iterator]() returns the iterator for the array, and it.next() is called repeatedly to get the next value in the sequence. When there are no more values, it.next() returns { value: undefined, done: true }.

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let myString = "Hello";

let it = myString[Symbol.iterator]();
console.log(it.next()); // { value: 'H', done: false }
console.log(it.next()); // { value: 'e', done: false }
console.log(it.next()); // { value: 'l', done: false }
console.log(it.next()); // { value: 'l', done: false }
console.log(it.next()); // { value: 'o', done: false }
console.log(it.next()); // { value: undefined, done: true }

In the example above, myString is a string, which is also an iterable object. To get its iterator, we call myString[Symbol.iterator](). The next() method of the iterator is then called repeatedly to get the next character in the string.

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let myObject = {
  a: 1,
  b: 2,
  c: 3
};

let it = Object.entries(myObject)[Symbol.iterator]();
console.log(it.next()); // { value: ['a', 1], done: false }
console.log(it.next()); // { value: ['b', 2], done: false }
console.log(it.next()); // { value: ['c', 3], done: false }
console.log(it.next()); // { value: undefined, done: true }

In the example above, myObject is an object, which is not an iterable object. However, we can use the Object.entries() method to get an array of its entries (key-value pairs), which is an iterable object. To get its iterator, we call Object.entries(myObject)[Symbol.iterator](). The next() method of the iterator is then called repeatedly to get the next entry in the array.