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 executingcallback
.
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:
- the value of the element
- the index of the element
- 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)
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:
- A simple for loop
- A for...of / for...in loops
Array.prototype.every()
Array.prototype.some()
Array.prototype.find()
Array.prototype.findIndex()
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.
forEach expects a synchronous function
forEach
does not wait for promises. Kindly make sure you are aware of the implications while using promises(or async functions) as forEach
callback.
Example Code
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) // Naively expected output: 14 // Actual output: 0
Examples
No operation for uninitialized values (sparse arrays)
const arraySparse = [1,3,,7] let numCallbackRuns = 0 arraySparse.forEach((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 copyItems = [] // before for (let i = 0; i < items.length; i++) { copyItems.push(items[i]) } // after items.forEach(function(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:
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((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.
Note: If passing the callback function uses an arrow function expression, the thisArg
parameter can 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 ECMAScript 5 Object.*
meta property functions.
function 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.
let 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()
.
function 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]] flatten(nested) // [1, 2, 3, 4, 5, 6, 7, 8, 9]
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Array.prototype.forEach' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
forEach | Chrome Full support 1 | Edge Full support 12 | Firefox Full support 1.5 | IE Full support 9 | Opera Full support 9.5 | Safari Full support 3 | WebView Android Full support ≤37 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support 10.1 | Safari iOS Full support 1 | Samsung Internet Android Full support 1.0 | nodejs Full support 0.1.100 |
Legend
- Full support
- Full support