JavaScript Array Iteration

JavaScript Array forEach()

The forEach() method calls a function (a callback function) once for each array element.

const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);

function myFunction(value, index, array) {
  txt += value + "<br>";
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

The example above uses only the value parameter. The example can be rewritten to:

const numbers = [45, 4, 9, 16, 25];
let txt = "";
numbers.forEach(myFunction);

function myFunction(value) {
  txt += value + "<br>";
}

JavaScript Array map()

The map() method creates a new array by performing a function on each array element.

The map() method does not execute the function for array elements without values.

The map() method does not change the original array.

This example multiplies each array value by 2:

const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(myFunction);

function myFunction(value, index, array) {
  return value * 2;
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

When a callback function uses only the value parameter, the index and array parameters can be omitted:

const numbers1 = [45, 4, 9, 16, 25];
const numbers2 = numbers1.map(myFunction);

function myFunction(value) {
  return value * 2;
}

JavaScript Array flatMap()

ES2019 added the Array flatMap() method to JavaScript.

The flatMap() method first maps all elements of an array and then creates a new array by flattening the array.

const myArr = [1, 2, 3, 4, 5, 6];
const newArr = myArr.flatMap((x) => x * 2);

Browser Support

JavaScript Array flatMap() is supported in all modern browsers since January 2020:

JavaScript Array filter()

The filter() method creates a new array with array elements that pass a test.

This example creates a new array from elements with a value larger than 18:

const numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(myFunction);

function myFunction(value, index, array) {
  return value > 18;
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

In the example above, the callback function does not use the index and array parameters, so they can be omitted:

const numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(myFunction);

function myFunction(value) {
  return value > 18;
}

JavaScript Array reduce()

The reduce() method runs a function on each array element to produce (reduce it to) a single value.

The reduce() method works from left-to-right in the array. See also reduceRight().

This example finds the sum of all numbers in an array:

const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);

function myFunction(total, value, index, array) {
  return total + value;
}

Note that the function takes 4 arguments:

• The total (the initial value / previously returned value)
• The item value
• The item index
• The array itself

The example above does not use the index and array parameters. It can be rewritten to:

const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction);

function myFunction(total, value) {
  return total + value;
}

The reduce() method can accept an initial value:

const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduce(myFunction, 100);

function myFunction(total, value) {
  return total + value;
}

JavaScript Array reduceRight()

The reduceRight() method runs a function on each array element to produce (reduce it to) a single value.

The reduceRight() works from right-to-left in the array. See also reduce().

This example finds the sum of all numbers in an array:

const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduceRight(myFunction);

function myFunction(total, value, index, array) {
  return total + value;
}

Note that the function takes 4 arguments:

• The total (the initial value / previously returned value)
• The item value
• The item index
• The array itself

The example above does not use the index and array parameters. It can be rewritten to:

const numbers = [45, 4, 9, 16, 25];
let sum = numbers.reduceRight(myFunction);

function myFunction(total, value) {
  return total + value;
}

JavaScript Array every()

The every() method checks if all array values pass a test.

This example checks if all array values are larger than 18:

const numbers = [45, 4, 9, 16, 25];
let allOver18 = numbers.every(myFunction);

function myFunction(value, index, array) {
  return value > 18;
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

When a callback function uses the first parameter only (value), the other parameters can be omitted:

const numbers = [45, 4, 9, 16, 25];
let allOver18 = numbers.every(myFunction);

function myFunction(value) {
  return value > 18;
}

JavaScript Array some()

The some() method checks if some array values pass a test.

This example checks if some array values are larger than 18:

const numbers = [45, 4, 9, 16, 25];
let someOver18 = numbers.some(myFunction);

function myFunction(value, index, array) {
  return value > 18;
}

Note that the function takes 3 arguments:

• The item value
• The item index
• The array itself

JavaScript Array.from()

The Array.from() method returns an Array object from any object with a length property or any iterable object.

Array.from("ABCDEFG");

Browser Support

from() is an ES6 feature (JavaScript 2015).

ES6 is fully supported in all modern browsers since June 2017:

from() is not supported in Internet Explorer.

JavaScript Array keys()

The Array.keys() method returns an Array Iterator object with the keys of an array.

const fruits = ["Banana", "Orange", "Apple", "Mango"];
const keys = fruits.keys();

for (let x of keys) {
  text += x + "<br>";
}

Browser Support

keys() is an ES6 feature (JavaScript 2015).

ES6 is fully supported in all modern browsers since June 2017:

keys() is not supported in Internet Explorer.

JavaScript Array entries()

const fruits = ["Banana", "Orange", "Apple", "Mango"];
const f = fruits.entries();

for (let x of f) {
  document.getElementById("demo").innerHTML += x;
}

The entries() method returns an Array Iterator object with key/value pairs:

[0, "Banana"]
[1, "Orange"]
[2, "Apple"]
[3, "Mango"]

The entries() method does not change the original array.

Browser Support

entries() is an ES6 feature (JavaScript 2015).

ES6 is fully supported in all modern browsers since June 2017:

entries() is not supported in Internet Explorer.

JavaScript Array with() Method

ES2023 added the Array with() method as a safe way to update elements in an array without altering the original array.

const months = ["Januar", "Februar", "Mar", "April"];
const myMonths = months.with(2, "March");

JavaScript Array Spread (...)

The ... operator expands an iterable (like an array) into more elements:

const q1 = ["Jan", "Feb", "Mar"];
const q2 = ["Apr", "May", "Jun"];
const q3 = ["Jul", "Aug", "Sep"];
const q4 = ["Oct", "Nov", "May"];

const year = [...q1, ...q2, ...q3, ...q4];

Browser Support

... is an ES6 feature (JavaScript 2015).

ES6 is fully supported in all modern browsers since June 2017:

... is not supported in Internet Explorer.