Array Iteration Methods
Array iteration methods operate on every array item.
| Col 1 | Col 2 |
|---|---|
| Array for...of Array for...in Array forEach() Array map() Array flatMap() Array filter() Array reduce() Array reduceRight() Array every() | Array some() Array from() Array keys() Array entries() Array with() Array Spread (...) Array Rest (...) |
Complete JavaScript Array Reference
Looping Array Values
JavaScript provides 3 simple statements for looping over each array items:
- for ... of (loops over entries)
- for ... in (loops over indexes)
- forEach (myFunction)
The for...of Loop (Recommended)
The for...of statements hands you the array values.
Example
// Create an Array
const cars = ['BMW', 'Volvo', 'Mini'];
// Iterate over the Array values
let text = "";
for (let x of cars) {
text += x + ",";
}
The for...in Loop (Not Recommended)
The for...in statements hands you the array indexes.
Example
// Create an Array
const cars = ['BMW', 'Volvo', 'Mini'];
// Iterate over the Array values
let text = "";
for (let x in cars) {
text += x + ",";
}
To make it return the elements you must use [x].
Example
// Create an Array
const cars = ['BMW', 'Volvo', 'Mini'];
// Iterate over the Array values
let text = "";
for (let x in cars) {
text += cars[x] + "
";
}
Note: Even though it works, using for...in for arrays is generally a bad idea. for...in was created to inspect Object properties (like: name, age, height), not Arrays.
JavaScript Array forEach()
The forEach() method calls a function (a callback function) for each array element.
Example
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:
Example
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:
Example
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:
Example
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.
Example
const myArr = [1, 2, 3, 4, 5, 6];
const newArr = myArr.flatMap(x => [x, x * 10]);
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 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:
Example
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 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
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
Since the example above does not use the index and array parameters, it can be rewritten to:
Example
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:
Example
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 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
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:
Example
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:
Example
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:
Example
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:
Example
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 iterable object
- Any object with a length property
Example
let text = "ABCDEFG";
Array.from(text);
Array.from() has an optional parameter which allows you to execute a function on each element of the new array:
Example
const myNumbers = [1,2,3,4];
const myArr = Array.from(myNumbers, (x) => x * 2);
JavaScript Array keys()
The Array.keys() method returns an Array Iterator object with the keys of an array.
Example
const fruits = ["Banana", "Orange", "Apple", "Mango"];
const keys = fruits.keys();
for (let x of keys) {
text += x + "<br>";
}
JavaScript Array entries()
Example
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.
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 months = ["Januar", "Februar", "Mar", "April"];
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 arr1 = [1, 2, 3];
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 q1 = ["Jan", "Feb", "Mar"];
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:
Example 3
const arr1 = [1, 2, 3];
const arr2 = [...arr1];
The spread operator (...) can be used to pass arguments to a function:
Example 4
const numbers = [23,55,21,87,56];
let minValue = Math.min(...numbers);
let maxValue = Math.max(...numbers);
JavaScript Array Rest (...)
The rest operator (...) allows us to destruct an array and collect the leftovers:
Examples
let a, rest;
const arr1 = [1,2,3,4,5,6,7,8];
[a, ...rest] = arr1;
Note: See Also: Array Tutorial Array Constructor Array Basic Methods Array Search Methods Array Sort Methods Array Reference
Note: 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.
