for...of - JavaScript 编辑

The for...of statement creates a loop iterating over iterable objects, including: built-in String, Array, array-like objects (e.g., arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. It invokes a custom iteration hook with statements to be executed for the value of each distinct property of the object.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

for (variable of iterable) {
  statement
}

variable

On each iteration a value of a different property is assigned to variable. variable may be declared with const, let, or var.

iterable

Object whose iterable properties are iterated.

Examples

Iterating over an Array

const iterable = [10, 20, 30];

for (const value of iterable) {
  console.log(value);
}
// 10
// 20
// 30

You can use let instead of const too, if you reassign the variable inside the block.

const iterable = [10, 20, 30];

for (let value of iterable) {
  value += 1;
  console.log(value);
}
// 11
// 21
// 31

Iterating over a String

const iterable = 'boo';

for (const value of iterable) {
  console.log(value);
}
// "b"
// "o"
// "o"

Iterating over a TypedArray

const iterable = new Uint8Array([0x00, 0xff]);

for (const value of iterable) {
  console.log(value);
}
// 0
// 255

Iterating over a Map

const iterable = new Map([['a', 1], ['b', 2], ['c', 3]]);

for (const entry of iterable) {
  console.log(entry);
}
// ['a', 1]
// ['b', 2]
// ['c', 3]

for (const [key, value] of iterable) {
  console.log(value);
}
// 1
// 2
// 3

Iterating over a Set

const iterable = new Set([1, 1, 2, 2, 3, 3]);

for (const value of iterable) {
  console.log(value);
}
// 1
// 2
// 3

Iterating over the arguments object

You can iterate over the arguments object to examine all of the parameters passed into a JavaScript function:

(function() {
  for (const argument of arguments) {
    console.log(argument);
  }
})(1, 2, 3);

// 1
// 2
// 3

Iterating over a DOM collection

Iterating over DOM collections like NodeList: the following example adds a read class to paragraphs that are direct descendants of an article:

// Note: This will only work in platforms that have
// implemented NodeList.prototype[Symbol.iterator]
const articleParagraphs = document.querySelectorAll('article > p');

for (const paragraph of articleParagraphs) {
  paragraph.classList.add('read');
}

Closing iterators

In for...of loops, abrupt iteration termination can be caused by break, throw or return. In these cases, the iterator is closed.

function* foo(){
  yield 1;
  yield 2;
  yield 3;
};

for (const o of foo()) {
  console.log(o);
  break; // closes iterator, execution continues outside of the loop
}
console.log('done');

Iterating over generators

You can also iterate over generators, i.e. functions generating an iterable object:

function* fibonacci() { // a generator function
  let [prev, curr] = [0, 1];
  while (true) {
    [prev, curr] = [curr, prev + curr];
    yield curr;
  }
}

for (const n of fibonacci()) {
  console.log(n);
  // truncate the sequence at 1000
  if (n >= 1000) {
    break;
  }
}

Do not reuse generators

Generators should not be re-used, even if the for...of loop is terminated early, for example via the break keyword. Upon exiting a loop, the generator is closed and trying to iterate over it again does not yield any further results.

const gen = (function *(){
  yield 1;
  yield 2;
  yield 3;
})();
for (const o of gen) {
  console.log(o);
  break;  // Closes iterator
}

// The generator should not be re-used, the following does not make sense!
for (const o of gen) {
  console.log(o); // Never called.
}

Iterating over other iterable objects

You can also iterate over an object that explicitly implements the iterable protocol:

const iterable = {
  [Symbol.iterator]() {
    return {
      i: 0,
      next() {
        if (this.i < 3) {
          return { value: this.i++, done: false };
        }
        return { value: undefined, done: true };
      }
    };
  }
};

for (const value of iterable) {
  console.log(value);
}
// 0
// 1
// 2

Difference between for...of and for...in

Both for...in and for...of statements iterate over something. The main difference between them is in what they iterate over.

The for...in statement iterates over the enumerable properties of an object, in an arbitrary order.

The for...of statement iterates over values that the iterable object defines to be iterated over.

The following example shows the difference between a for...of loop and a for...in loop when used with an Array.

Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};

const iterable = [3, 5, 7];
iterable.foo = 'hello';

for (const i in iterable) {
  console.log(i); // logs 0, 1, 2, "foo", "arrCustom", "objCustom"
}

for (const i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i); // logs 0, 1, 2, "foo"
  }
}

for (const i of iterable) {
  console.log(i); // logs 3, 5, 7
}

Let us look into the above code step by step.

Object.prototype.objCustom = function() {};
Array.prototype.arrCustom = function() {};

const iterable = [3, 5, 7];
iterable.foo = 'hello';

Every object will inherit the objCustom property and every object that is an Array will inherit the arrCustom property since these properties have been added to Object.prototype and Array.prototype, respectively. The object iterable inherits the properties objCustom and arrCustom because of inheritance and the prototype chain.

for (const i in iterable) {
  console.log(i); // logs 0, 1, 2, "foo", "arrCustom", "objCustom"
}

This loop logs only enumerable properties of the iterable object, in arbitrary order. It doesn't log array elements 3, 5, 7 or hello because those are not enumerable properties, in fact they are not properties at all, they are values. It logs array indexes as well as arrCustom and objCustom, which are. If you're not sure why these properties are iterated over, there's a more thorough explanation of how array iteration and for...in work.

for (const i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i); // logs 0, 1, 2, "foo"
  }
}

This loop is similar to the first one, but it uses hasOwnProperty() to check if the found enumerable property is the object's own, i.e. not inherited. If it is, the property is logged. Properties 0, 1, 2 and foo are logged because they are own properties (not inherited). Properties arrCustom and objCustom are not logged because they are inherited.

for (const i of iterable) {
  console.log(i); // logs 3, 5, 7
}

This loop iterates and logs values that iterable, as an iterable object, defines to be iterated over. The object's elements 3, 5, 7 are shown, but none of the object's properties.

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'for...of statement' in that specification.

Browser compatibility

BCD tables only load in the browser

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

See also

• Array.prototype.forEach()
• Map.prototype.forEach()
• Object.entries() – Useful when using for...of over an object.