The forEach()
method executes a provided function once for each array element.
const arr = ['a', 'b', 'c']; arr.forEach(function(element) { console.log(element); }); // a // b // c
arr.forEach(function callback(currentValue[, index[, array]]) { //your iterator }[, thisArg]);
callback
currentValue
index
Optional
array
Optional
forEach()
is being applied to.thisArg
Optional
Value to use as this
(i.e the reference Object
) when executing callback
.
forEach()
executes the provided callback
once for each element present in the array in ascending order. It is not invoked for index properties that have been deleted or are uninitialized (i.e. on sparse arrays).
callback
is invoked with three arguments:
If a thisArg
parameter is provided to forEach()
, it will be used as callback's this
value. Otherwise, the value undefined
will be used as its this
value. The this
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 that are appended to the array after the call to forEach()
begins will not be visited by callback
. If the values of existing elements of the array are changed, the value 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 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 (although callback
, if invoked, 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. Use a plain loop or for...of instead. If you are testing the array elements for a predicate and need a Boolean return value, you can use every()
or some()
instead. If available, the new methods find()
or findIndex()
can be used for early termination upon true predicates as well.
before
const items = ['item1', 'item2', 'item3']; const copy = []; for (let i=0; i<items.length; i++) { copy.push(items[i]) }
after
const items = ['item1', 'item2', 'item3']; const copy = []; items.forEach(function(item){ copy.push(item) });
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
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, for use as its this
value.
If passing the function argument using an arrow function expression the thisArg
parameter can be omitted as arrow functions lexically bind the this
value.
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
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, which results 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.
var words = ['one', 'two', 'three', 'four']; words.forEach(function(word) { console.log(word); if (word === 'two') { words.shift(); } }); // one // two // four
forEach()
was added to the ECMA-262 standard in the 5th edition; as such it may not be present in other implementations of the standard. You can work around this by inserting the following code at the beginning of your scripts, allowing use of forEach()
in implementations that don't natively support it. This algorithm is exactly the one specified in ECMA-262, 5th edition, assuming Object
and TypeError
have their original values and that callback.call()
evaluates to the original value of Function.prototype.call()
.
// Production steps of ECMA-262, Edition 5, 15.4.4.18 // Reference: http://es5.github.io/#x15.4.4.18 if (!Array.prototype.forEach) { Array.prototype.forEach = function(callback/*, thisArg*/) { var T, k; if (this == null) { throw new TypeError('this is null or not defined'); } // 1. Let O be the result of calling toObject() passing the // |this| value as the argument. var O = Object(this); // 2. Let lenValue be the result of calling the Get() internal // method of O with the argument "length". // 3. Let len be toUint32(lenValue). var len = O.length >>> 0; // 4. If isCallable(callback) is false, throw a TypeError exception. // See: http://es5.github.com/#x9.11 if (typeof callback !== 'function') { throw new TypeError(callback + ' is not a function'); } // 5. If thisArg was supplied, let T be thisArg; else let // T be undefined. if (arguments.length > 1) { T = arguments[1]; } // 6. Let k be 0. k = 0; // 7. Repeat while k < len. while (k < len) { var kValue; // a. Let Pk be ToString(k). // This is implicit for LHS operands of the in operator. // b. Let kPresent be the result of calling the HasProperty // internal method of O with argument Pk. // This step can be combined with c. // c. If kPresent is true, then if (k in O) { // i. Let kValue be the result of calling the Get internal // method of O with argument Pk. kValue = O[k]; // ii. Call the Call internal method of callback with T as // the this value and argument list containing kValue, k, and O. callback.call(T, kValue, k, O); } // d. Increase k by 1. k++; } // 8. return undefined. }; }
Specification | Status | Comment |
---|---|---|
ECMAScript 5.1 (ECMA-262) The definition of 'Array.prototype.forEach' in that specification. | Standard | Initial definition. Implemented in JavaScript 1.6. |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array.prototype.forEach' in that specification. | Standard | |
ECMAScript Latest Draft (ECMA-262) The definition of 'Array.prototype.forEach' in that specification. | Draft |
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | Yes | Yes | 1.5 | 9 | Yes | Yes |
Feature | Android webview | Chrome for Android | Edge mobile | Firefox for Android | Opera Android | iOS Safari | Samsung Internet |
---|---|---|---|---|---|---|---|
Basic support | Yes | Yes | Yes | 4 | Yes | Yes | ? |
Array.prototype.find()
Array.prototype.findIndex()
Array.prototype.map()
Array.prototype.every()
Array.prototype.some()
Map.prototype.forEach()
Set.prototype.forEach()
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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