Array.prototype.forEach()

forEach() metodu dizideki her eleman için verilen metodu çalıştırır.

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.

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

callback

Aşağıdaki üç parametreyi alan ve dizinin her elemanı için çalışan fonksiyon.

currentValueOptional

İşlenmekte olan dizi elemanı.

indexOptional

İşlenmekte olan dizi elemanının indeksi, yani dizideki sırası.

arrayOptional

forEach() in uygulanmakta olduğu dizi.

thisArg Optional

callback fonksiyonu çağırılırken, fonksiyon içerisinde this yerine kullanılılabilecek değer.

undefined.

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).

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

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.

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.
  };
}

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