forEach()
metodu dizideki her eleman için verilen metodu çalıştırır.
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.
Bu interaktif örneğin kaynağını GitHub deposunda bulabilirsiniz. Eğer bu interaktif projelere katkı sağlamak isterseni, https://github.com/mdn/interactive-examples adresindeki depoyu klonlayın ve bize pull request'i gönderin.
Syntax
arr.forEach(function callback(currentValue[, index[, array]]) { //your iterator }[, thisArg]);
Parametreler
callback
- Aşağıdaki üç parametreyi alan ve dizinin her elemanı için çalışan fonksiyon.
-
currentValue
Optional- İşlenmekte olan dizi elemanı.
index
Optional- İşlenmekte olan dizi elemanının indeksi, yani dizideki sırası.
array
OptionalforEach()
in uygulanmakta olduğu dizi.
thisArg
Optional-
callback
fonksiyonu çağırılırken, fonksiyon içerisindethis
yerine kullanılılabilecek değer.
Dönüş Değeri
Tanım
forEach()
tanımlanmış olan callback
fonksiyonunu dizideki her eleman için bir kere olmak üzere, indeks sırasına göre artan şekilde çalıştırır. Silinmiş ya da tanımsız olan elemanlar için fonksiyon çalışmaz (örnek: seyrek diziler).
Dizinin
callback
çağırılırken aşağıdaki üç parametre kullanılır:
- Dizi elemanının değeri
- Dizi elemanının indeksi
- Döngünün gerçekleştiği dizinin kendisi
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.
Early termination may be accomplished with:
- A simple loop
- A for...of loop
Array.prototype.every()
Array.prototype.some()
Array.prototype.find()
Array.prototype.findIndex()
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.
Examples
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.
};
}
Specifications
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 (ECMA-262) The definition of 'Array.prototype.forEach' in that specification. |
Living Standard |
Browser compatibility
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