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Array.prototype.forEach()

The forEach() method executes a provided function once for each array element.

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Syntax

// Arrow function
forEach((element) => { /* … */ })
forEach((element, index) => { /* … */ })
forEach((element, index, array) => { /* … */ })

// Callback function
forEach(callbackFn)
forEach(callbackFn, thisArg)

// Inline callback function
forEach(function(element) { /* … */ })
forEach(function(element, index) { /* … */ })
forEach(function(element, index, array){ /* … */ })
forEach(function(element, index, array) { /* … */ }, thisArg)

Parameters

callbackFn

Function to execute on each element.

The function is called with the following arguments:

element

The current element being processed in the array.

index

The index of element in the array.

array

The array forEach() was called upon.

thisArg Optional

Value to use as this when executing callbackFn.

Return value

undefined.

Description

forEach() calls a provided callbackFn function once for each element in an array in ascending index order. It is not invoked for index properties that have been deleted or are uninitialized. (For sparse arrays, see example below.)

callbackFn is invoked with three arguments:

  1. the value of the element
  2. the index of the element
  3. the Array object being traversed

If a thisArg parameter is provided to forEach(), it will be used as callback's this value. The thisArg value ultimately observable by callbackFn is determined according to the usual rules for determining the this seen by a function.

The range of elements processed by forEach() is set before the first invocation of callbackFn. Elements which are assigned to indexes already visited, or to indexes outside the range, will not be visited by callbackFn. If existing elements of the array are changed or deleted, their value as passed to callbackFn will be the value at the time forEach() visits them; elements that are deleted before being visited are not visited. If elements that are already visited are removed (e.g. using shift()) during the iteration, later elements will be skipped. (See this example, below.)

Warning: Concurrent modification of the kind described in the previous paragraph frequently leads to hard-to-understand code and is generally to be avoided (except in special cases).

forEach() executes the callbackFn function once for each array element; unlike map() or reduce() it always returns the value undefined and is not chainable. The typical use case is to execute side effects at the end of a chain.

forEach() does not mutate the array on which it is called. (However, callbackFn may do so)

Note: There is no way to stop or break a forEach() loop other than by throwing an exception. If you need such behavior, the forEach() method is the wrong tool.

Early termination may be accomplished with:

Array methods: every(), some(), find(), and findIndex() test the array elements with a predicate returning a truthy value to determine if further iteration is required.

Note: forEach expects a synchronous function.

forEach does not wait for promises. Make sure you are aware of the implications while using promises (or async functions) as forEach callback.

const ratings = [5, 4, 5];
let sum = 0;

const sumFunction = async (a, b) => a + b;

ratings.forEach(async (rating) => {
  sum = await sumFunction(sum, rating);
});

console.log(sum);
// Naively expected output: 14
// Actual output: 0

Examples

No operation for uninitialized values (sparse arrays)

const arraySparse = [1, 3, /* empty */, 7];
let numCallbackRuns = 0;

arraySparse.forEach((element) => {
  console.log({ element });
  numCallbackRuns++;
});

console.log({ numCallbackRuns });

// 1
// 3
// 7
// numCallbackRuns: 3
// comment: as you can see the missing value between 3 and 7 didn't invoke callback function.

Converting a for loop to forEach

const items = ['item1', 'item2', 'item3'];
const copyItems = [];

// before
for (let i = 0; i < items.length; i++) {
  copyItems.push(items[i]);
}

// after
items.forEach((item) => {
  copyItems.push(item);
});

Printing the contents of an array

Note: In order to display the content of an array in the console, you can use console.table(), which prints a formatted version of the array.

The following example illustrates an alternative approach, using forEach().

The following code logs a line for each element in an array:

const logArrayElements = (element, index, array) => {
  console.log(`a[${index}] = ${element}`);
};

// Notice that index 2 is skipped, since there is no item at
// that position in the array.
[2, 5, , 9].forEach(logArrayElements);
// logs:
// a[0] = 2
// a[1] = 5
// a[3] = 9

Using thisArg

The following (contrived) example updates an object's properties from each entry in the array:

class Counter {
  constructor() {
    this.sum = 0;
    this.count = 0;
  }
  add(array) {
    // Only function expressions will have its own this binding
    array.forEach(function countEntry(entry) {
      this.sum += entry;
      ++this.count;
    }, this);
  }
}

const obj = new Counter();
obj.add([2, 5, 9]);
console.log(obj.count); // 3
console.log(obj.sum); // 16

Since the thisArg parameter (this) is provided to forEach(), it is passed to callback each time it's invoked. The callback uses it as its this value.

Note: If passing the callback function used an arrow function expression, the thisArg parameter could be omitted, since all arrow functions lexically bind the this value.

An object copy function

The following code creates a copy of a given object.

There are different ways to create a copy of an object. The following is just one way and is presented to explain how Array.prototype.forEach() works by using Object.* utility functions.

const copy = (obj) => {
  const copy = Object.create(Object.getPrototypeOf(obj));
  const propNames = Object.getOwnPropertyNames(obj);
  propNames.forEach((name) => {
    const desc = Object.getOwnPropertyDescriptor(obj, name);
    Object.defineProperty(copy, name, desc);
  });
  return copy;
};

const obj1 = { a: 1, b: 2 };
const obj2 = copy(obj1); // obj2 looks like obj1 now

Modifying the array during iteration

The following example logs one, two, four.

When the entry containing the value two is reached, the first entry of the whole array is shifted off—resulting in all remaining entries moving up one position. Because element four is now at an earlier position in the array, three will be skipped.

forEach() does not make a copy of the array before iterating.

const words = ['one', 'two', 'three', 'four'];
words.forEach((word) => {
  console.log(word);
  if (word === 'two') {
    words.shift(); //'one' will delete from array
  }
}); // one // two // four

console.log(words); // ['two', 'three', 'four']

Flatten an array

The following example is only here for learning purpose. If you want to flatten an array using built-in methods you can use Array.prototype.flat().

const flatten = (arr) => {
  const result = [];
  arr.forEach((i) => {
    if (Array.isArray(i)) {
      result.push(...flatten(i));
    } else {
      result.push(i);
    }
  });
  return result;
}

// Usage
const nested = [1, 2, 3, [4, 5, [6, 7], 8, 9]];
console.log(flatten(nested)); // [1, 2, 3, 4, 5, 6, 7, 8, 9]

Specifications

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
forEach
1
12
1.5
9
9.5
3
≤37
18
4
10.1
1
1.0
1.0
0.10.0

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach