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Phương thức forEach() sẽ thực thi một hàm khi duyệt qua từng phần tử của mảng.

Cú pháp

arr.forEach(function callback(currentValue[, index[, array]]) {
    //your iterator
}[, thisArg]);

Parameters

callback
Hàm sẽ thực thi lên từng phần tử của mảng được gọi, hàm này nhận 3 tham số:
currentValueOptional
Giá trị của phần tử đang được duyệt.
indexOptional
Chỉ mục của phần tử đang được duyệt.
arrayOptional
Mảng mà hàm forEach() đang duyệt.
thisArg Optional

Giá trị được gán cho từ khóa this bên trong hàm callback khi được thực thi.

Giá trị trả về

undefined.

Miêu tả chi tiết

forEach() thực thi hàm callback trong lúc duyệt tới từng phần tử của mảng theo thứ tự tăng dần. Nó sẽ không được gọi cho vị trí index đã bị xóa hoặc không được khởi tạo (đối với mảng thưa - sparse arrays).

callback được gọi với 3 tham số:

  • giá trị của phần tử
  • index của phần tử
  • mảng đang được duyệt

Nếu tham số thisArg được truyền vào cho forEach(), nó sẽ được dùng cho từ khóa this trong hàm callback. Nếu không, giá trị undefined sẽ được dùng cho từ khóa this. Tóm lại, giá trị của từ khóa this trong hàm callback được xác định tuân theo các quy tắc thông thường để xác định this trong một hàm.

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.

Early termination may be accomplished with:

The other 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.

Ví dụ

Converting a for loop to forEach

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)
});

 

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, 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.

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, 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

Polyfill

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.
  };
}

Đặc tả

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  

Trình duyệt tương thích

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
Basic supportChrome Full support YesEdge Full support YesFirefox Full support 1.5IE Full support 9Opera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 4Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support Yes

Legend

Full support  
Full support

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