Iterators Back

Since ECMAScript 2015(ES6), an object, like Array, Map, Set, String, Int32Array, Uint32Array, etc., has been deemed iterable if it has an implementation of Symbol.iterator property:

console.log('123'[Symbol.iterator]); /** => ƒ [Symbol.iterator]() { [native code] } */

The property has also ben used by for...of statements:

const arr = [1, 'string', false];
for (const value of arr) {
    console.log(value); /** 1, 'string', false */

Similar to for...of statements, statements are mainly used for accessing keys of iterable objects.

When targeting an ES5 or ES3 within TypeScript, iterators are only allowed on values of Array type, and the compiler will automatically generate a simple for loop for a for...of loop, for instance:

let numbers = [1, 2, 3];
for (let num of numbers) {

will be generated as:

var numbers = [1, 2, 3];
for (var _i = 0; _i < numbers.length; _i++) {
    var num = numbers[_i];
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