Array.prototype.forEach()

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

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

arr.forEach(callback(currentValue [, index [, array]])[, thisArg])

Parameters

callback

Function to execute on each element. It accepts between one and three arguments:

currentValue

The current element being processed in the array.

index Optional

The index currentValue in the array.

array Optional

The array forEach() was called upon.

thisArg Optional

Value to use as this when executing callback.

Return value

undefined.

Description

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

callback 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 callback 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 callback. Elements which are appended to the array after the call to forEach() begins will not be visited by callback. If existing elements of the array are changed or deleted, their value as passed to callback 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.)

forEach() executes the callback 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, callback may do so)

Examples

No operation for uninitialized values (sparse arrays)

const arraySparse = [1,3,,7]
let numCallbackRuns = 0

arraySparse.forEach(function(element){
  console.log(element)
  numCallbackRuns++
})

console.log("numCallbackRuns: ", 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 copy = []

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

// after
items.forEach(function(item){
  copy.push(item)
})

Printing the contents of an array

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

function 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:

function Counter() {
  this.sum = 0
  this.count = 0
}
Counter.prototype.add = function(array) {
  array.forEach(function(entry) {
    this.sum += entry
    ++this.count
  }, this)
  // ^---- Note
}

const obj = new Counter()
obj.add([2, 5, 9])
obj.count
// 3 
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.

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 ECMAScript 5 Object.* meta property functions.

function copy(obj) {
  const copy = Object.create(Object.getPrototypeOf(obj))
  const propNames = Object.getOwnPropertyNames(obj)

  propNames.forEach(function(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

If the array is modified during iteration, other elements might be skipped.

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.

let words = ['one', 'two', 'three', 'four']
words.forEach(function(word) {
  console.log(word)
  if (word === 'two') {
    words.shift()
  }
})
// one
// two
// 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() (which is expected to be part of ES2019, and is already implemented in some browsers).

/**
 * Flattens passed array in one dimensional array
 *
 * @params {array} arr
 * @returns {array}
 */
function flatten(arr) {
  const result = []

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

// Usage
const problem = [1, 2, 3, [4, 5, [6, 7], 8, 9]]

flatten(problem) // [1, 2, 3, 4, 5, 6, 7, 8, 9]

Note on using Promises or async functions

let ratings = [5, 4, 5]

let sum = 0

let sumFunction = async function (a, b)
{
    return a + b
} 

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

console.log(sum)
// Expected output: 14
// Actual output: 0

Specifications

Browser compatibility

See also

• Array.prototype.find()
• Array.prototype.findIndex()
• Array.prototype.map()
• Array.prototype.filter()
• Array.prototype.every()
• Array.prototype.some()
• Map.prototype.forEach()
• Set.prototype.forEach()