Javascript foreach value in array

JavaScript Array forEach()

The forEach() method calls a function for each element in an array.

The forEach() method is not executed for empty elements.

See Also:

Syntax

Parameters

function()	Required. A function to run for each array element.
currentValue	Required. The value of the current element.
index	Optional. The index of the current element.
arr	Optional. The array of the current element.
thisValue	Optional. Default undefined . A value passed to the function as its this value.

Return Value

More Examples

let sum = 0;
const numbers = [65, 44, 12, 4];
numbers.forEach(myFunction);

function myFunction(item) sum += item;
>

const numbers = [65, 44, 12, 4];
numbers.forEach(myFunction)

function myFunction(item, index, arr) arr[index] = item * 10;
>

Related Pages:

Browser Support

forEach() is an ECMAScript5 (ES5) feature.

ES5 (JavaScript 2009) fully supported in all browsers:

Chrome	Edge	Firefox	Safari	Opera	IE
Yes	Yes	Yes	Yes	Yes	9-11

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Array.prototype.forEach()

The forEach() method executes a provided function once for each array element.

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Syntax

forEach(callbackFn) forEach(callbackFn, thisArg) 

Parameters

A function to execute for each element in the array. Its return value is discarded. The function is called with the following arguments:

The current element being processed in the array.

The index of the current element being processed in the array.

The array forEach() was called upon.

A value to use as this when executing callbackFn . See iterative methods.

Return value

Description

The forEach() method is an iterative method. It calls a provided callbackFn function once for each element in an array in ascending-index order. Unlike map() , forEach() always returns undefined and is not chainable. The typical use case is to execute side effects at the end of a chain.

callbackFn is invoked only for array indexes which have assigned values. It is not invoked for empty slots in sparse arrays.

forEach() does not mutate the array on which it is called, but the function provided as callbackFn can. Note, however, that the length of the array is saved before the first invocation of callbackFn . Therefore:

• callbackFn will not visit any elements added beyond the array’s initial length when the call to forEach() began.
• Changes to already-visited indexes do not cause callbackFn to be invoked on them again.
• If an existing, yet-unvisited element of the array is changed by callbackFn , its value passed to the callbackFn will be the value at the time that element gets visited. Deleted elements are not visited.

Warning: Concurrent modifications of the kind described above frequently lead to hard-to-understand code and are generally to be avoided (except in special cases).

The forEach() method is generic. It only expects the this value to have a length property and integer-keyed properties.

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 looping statements like for , for. of , and for. in . Array methods like every() , some() , find() , and findIndex() also stops iteration immediately when further iteration is not necessary.

forEach() expects a synchronous function — it does not wait for promises. Make sure you are aware of the implications while using promises (or async functions) as forEach callbacks.

const ratings = [5, 4, 5]; let sum = 0; const sumFunction = async (a, b) => a + b; ratings.forEach(async (rating) =>  sum = await sumFunction(sum, rating); >); console.log(sum); // Naively expected output: 14 // Actual output: 0 

To run a series of asynchronous operations sequentially or concurrently, see promise composition.

Examples

Using forEach() on sparse arrays

const arraySparse = [1, 3, /* empty */, 7]; let numCallbackRuns = 0; arraySparse.forEach((element) =>  console.log( element >); numCallbackRuns++; >); console.log( numCallbackRuns >); // // // // 

The callback function is not invoked for the missing value at index 2.

Converting a for loop to forEach

const items = ["item1", "item2", "item3"]; const copyItems = []; // before for (let i = 0; i  items.length; i++)  copyItems.push(items[i]); > // after items.forEach((item) =>  copyItems.push(item); >); 

Printing the contents of an array

Note: In order to display the content of an array in the console, you can use console.table() , which prints a formatted version of the array.

The following example illustrates an alternative approach, using forEach() .

The following code logs a line for each element in an array:

const 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:

class Counter  constructor()  this.sum = 0; this.count = 0; > add(array)  // Only function expressions will have its own this binding array.forEach(function countEntry(entry)  this.sum += entry; ++this.count; >, this); > > const obj = new Counter(); obj.add([2, 5, 9]); console.log(obj.count); // 3 console.log(obj.sum); // 16 

Since the thisArg parameter ( this ) is provided to forEach() , it is passed to callback each time it’s invoked. The callback uses it as its this value.

Note: If passing the callback function used an arrow function expression, the thisArg parameter could be omitted, since all 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 Object.* utility functions.

const copy = (obj) =>  const copy = Object.create(Object.getPrototypeOf(obj)); const propNames = Object.getOwnPropertyNames(obj); propNames.forEach((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 

Modifying the array during iteration

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—resulting 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.

const words = ["one", "two", "three", "four"]; words.forEach((word) =>  console.log(word); if (word === "two")  words.shift(); //'one' will delete from array > >); // one // two // four console.log(words); // ['two', 'three', 'four'] 

Flatten an array

The following example is only here for learning purpose. If you want to flatten an array using built-in methods, you can use Array.prototype.flat() .

const flatten = (arr) =>  const result = []; arr.forEach((item) =>  if (Array.isArray(item))  result.push(. flatten(item)); > else  result.push(item); > >); return result; >; // Usage const nested = [1, 2, 3, [4, 5, [6, 7], 8, 9]]; console.log(flatten(nested)); // [1, 2, 3, 4, 5, 6, 7, 8, 9] 

Calling forEach() on non-array objects

The forEach() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length .

const arrayLike =  length: 3, 0: 2, 1: 3, 2: 4, 3: 5, // ignored by forEach() since length is 3 >; Array.prototype.forEach.call(arrayLike, (x) => console.log(x)); // 2 // 3 // 4 

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Polyfill of Array.prototype.forEach in core-js
• Indexed collections
• Array
• Array.prototype.find()
• Array.prototype.map()
• Array.prototype.filter()
• Array.prototype.every()
• Array.prototype.some()
• TypedArray.prototype.forEach()
• Map.prototype.forEach()
• Set.prototype.forEach()

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