Array.prototype.forEach()

callbackFn

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

element

The current element being processed in the array.

index

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

array

The array forEach() was called upon.

thisArg Optional

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

None (undefined).

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. Read the iterative methods section for more information about how these methods work in general.

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

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.

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

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 have their own this bindings.
    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.

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

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]

The array argument is useful if you want to access another element in the array, especially when you don't have an existing variable that refers to the array. The following example first uses filter() to extract the positive values and then uses forEach() to log its neighbors.

const numbers = [3, -1, 1, 4, 1, 5];
numbers
  .filter((num) => num > 0)
  .forEach((num, idx, arr) => {
    // Without the arr argument, there's no way to easily access the
    // intermediate array without saving it to a variable.
    console.log(arr[idx - 1], num, arr[idx + 1]);
  });
// undefined 3 1
// 3 1 4
// 1 4 1
// 4 1 5
// 1 5 undefined

const arraySparse = [1, 3, /* empty */, 7];
let numCallbackRuns = 0;

arraySparse.forEach((element) => {
  console.log({ element });
  numCallbackRuns++;
});

console.log({ numCallbackRuns });

// { element: 1 }
// { element: 3 }
// { element: 7 }
// { numCallbackRuns: 3 }

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

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

• Polyfill of Array.prototype.forEach in core-js
• Indexed collections guide
• 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()