JavaScript Array Iterations
Array Iteration Methods
Array iteration methods operate on every array item.
See Also:
JavaScript Array forEach()
The forEach() method calls a function (a callback function) once for each array element.
Example
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:
Example
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:
Example
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:
Example
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.
Example
const newArr = myArr.flatMap((x) => x * 2);
Browser Support
flatMap() is an ECMAScript 2019 feature.
ES2019 is supported in all modern browsers since January 2020:
| Chrome 66 |
Edge 79 |
Firefox 61 |
Safari 12 |
Opera 50 |
| Apr 2018 | Jan 2020 | Jun 2018 | Sep 2018 | May 2018 |
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:
Example
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:
Example
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 a single value.
The reduce() method works from left-to-right in the array. See also reduceRight().
Note
The reduce() method does not reduce the original array.
This example finds the sum of all numbers in an array:
Example
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
Since the example above does not use the index and array parameters, it can be rewritten to:
Example
let sum = numbers.reduce(myFunction);
function myFunction(total, value) {
return total + value;
}
The reduce() method can accept an initial value:
Example
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 a single value.
The reduceRight() works from right-to-left in the array. See also reduce().
Note
The reduceRight() method does not reduce the original array.
This example finds the sum of all numbers in an array:
Example
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:
Example
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:
Example
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:
Example
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:
Example
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 iterable object
Any object with a length property
Array.from() has an optional parameter which allows you to execute a function
on each element of the new array:
Example
Create an Array from an Array:
const myArr = Array.from(myNumbers, (x) => x * 2);
Browser Support
from() is an ES6 feature.
ES6 is fully supported in all modern browsers since June 2017:
| Chrome 51 |
Edge 15 |
Firefox 54 |
Safari 10 |
Opera 38 |
| May 2016 | Apr 2017 | Jun 2017 | Sep 2016 | Jun 2016 |
JavaScript Array keys()
The Array.keys() method returns an Array Iterator object with the keys of an array.
Example
Create an Array Iterator object, containing the keys of the array:
const keys = fruits.keys();
for (let x of keys) {
text += x + "<br>";
}
Browser Support
keys() is an ES6 feature.
ES6 is fully supported in all modern browsers since June 2017:
| Chrome 51 |
Edge 15 |
Firefox 54 |
Safari 10 |
Opera 38 |
| May 2016 | Apr 2017 | Jun 2017 | Sep 2016 | Jun 2016 |
JavaScript Array entries()
Example
Create an Array Iterator, and then iterate over the key/value pairs:
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.
ES6 is fully supported in all modern browsers since June 2017:
| Chrome 51 |
Edge 15 |
Firefox 54 |
Safari 10 |
Opera 38 |
| May 2016 | Apr 2017 | Jun 2017 | Sep 2016 | Jun 2016 |
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.
Example
const myMonths = months.with(2, "March");
JavaScript Array Spread (...)
The ... operator expands an array into individual elements.
This can be used join arrays:
Example 1
const arr2 = [4, 5, 6];
const arr3 = [...arr1, ...arr2];
In the example above, ...arr1 expands arr1 into single elements, ...arr2 expands arr2 into single elements, and arr3 is constructed using ...arr1 and ...arr2.
Example 2
const q2 = ["Apr", "May", "Jun"];
const q3 = ["Jul", "Aug", "Sep"];
const q4 = ["Oct", "Nov", "Des"];
const year = [...q1, ...q2, ...q3, ...q4];
The spread operator (...) can be used to copy an array:
The spread operator (...) can be used to pass arguments to a function:
Example 4
let minValue = Math.min(...numbers);
let maxValue = Math.max(...numbers);
Browser Support
... (spread) is an ES6 feature.
ES6 is fully supported in all modern browsers since June 2017:
| Chrome 51 |
Edge 15 |
Firefox 54 |
Safari 10 |
Opera 38 |
| May 2016 | Apr 2017 | Jun 2017 | Sep 2016 | Jun 2016 |
JavaScript Array Rest (...)
The rest operator (...) allows us to destruct an array and collect the leftovers:
Browser Support
... (rest) is an ECMAScript 2018 feature.
ES2018 is supported in all modern browsers since January 2020:
| Chrome 64 |
Edge 79 |
Firefox 58 |
Safari 12 |
Opera 51 |
| Jan 2018 | Jan 2020 | Jan 2018 | Sep 2018 | Feb 2018 |
Complete JavaScript Reference
For a complete reference to all JavaScript properties and methods, with full descriptions and many examples, go to:
W3Schools' Full JavaScript Reference.
The reference inludes all JavaScript updates from 1999 to 2025.