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Rezumat
Metoda forEach()
execută o funcție dată pentru fiecare element din array.
Sintaxă
arr.forEach(callback[, thisArg])
Parametri
callback
- Funcția care se va executa pe fiecare element.
thisArg
- Obiect care va fi folosit ca
this
la execuțiacallback
.
Descriere
forEach
execută funcția callback
dată ca parametru o dată pentru fiecare element prezent în listă, în ordine crescătoare. Nu este invocată pentru indecși care au fost șterși sau ocoliți. Cu toate acestea, este executată pentru elemente care sunt prezente și au valoarea undefined
.
callback
is invoked with three arguments:
- the element value
- the element index
- the array being traversed
If a thisArg
parameter is provided to forEach
, it will be passed to callback
when invoked, for use as its this
value. Otherwise, the value undefined
will be passed for use 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.
Note: There is no way to stop or break a forEach loop. The solution is to use Array.every
or Array.some
. See example below.
forEach
executes the callback
function once for each array element; unlike every
it, always returns the value and some
undefined
.
Examples
Example: 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); } // Note ellision, there is no member at 2 so it isn't visited [2, 5, , 9].forEach(logArrayElements); // logs: // a[0] = 2 // a[1] = 5 // a[3] = 9
Example: Breaking a loop
The following code uses Array.prototype.every
to log the content of an array and stop when it reaches a value higher than the given THRESHOLD
.
var THRESHOLD = 12; var v = [5, 2, 16, 4, 3, 18, 20]; var res; res = v.every(function(element, index, array) { console.log('element:', element); if (element >= THRESHOLD) { return false; } return true; }); console.log('res:', res); // logs: // element: 5 // element: 2 // element: 16 // res: false res = v.some(function(element, index, array) { console.log('element:', element); if (element >= THRESHOLD) { return true; } return false; }); console.log('res:', res); // logs: // element: 5 // element: 2 // element: 16 // res: true
Example: 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(o) { var copy = Object.create(Object.getPrototypeOf(o)); var propNames = Object.getOwnPropertyNames(o); propNames.forEach(function(name) { var desc = Object.getOwnPropertyDescriptor(o, name); Object.defineProperty(copy, name, desc); }); return copy; } var o1 = { a: 1, b: 2 }; var o2 = copy(o1); // o2 looks like o1 now
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: https://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: https://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 = thisArg; } // 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 |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | (Yes) | 1.5 (1.8) | 9 | (Yes) | (Yes) |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | (Yes) | (Yes) | 1.0 (1.8) | (Yes) | (Yes) | (Yes) |