JavaScript Array Iteration

Guest · Post by **Guest** » Fri Oct 27, 2023 9:07 am

JavaScript Array Iteration

Array iteration methods operate on every array item.

JavaScript Array forEach()
The forEach() method calls a function (a callback function) once 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>";
}

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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>";
}

Try it Yourself »

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;
}

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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;
}

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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 * 2);

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

Chrome 69
Edge 79
Firefox 62
Safari 12
Opera 56

Sep 2018
Jan 2020
Sep 2018
Sep 2018
Sep 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 numbers = [45, 4, 9, 16, 25];
const over18 = numbers.filter(myFunction);
function myFunction(value, index, array) {  return value > 18;}
Try it Yourself »

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;}
Try it Yourself »

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().

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;}
Try it Yourself »

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.reduce(myFunction);
function myFunction(total, value) {
return total + value;}
Try it Yourself »

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;}
Try it Yourself »

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().

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;}
Try it Yourself »

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;}
Try it Yourself »

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;}
Try it Yourself »

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;}
Try it Yourself »

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;}
Try it Yourself »

Note that the function takes 3 arguments:

The item value
The item index
The array itself

JavaScript Array indexOf()
The indexOf() method searches an array for an element value and returns its position.

Note: The first item has position 0, the second item has position 1, and so on.

Example
Search an array for the item "Apple":

const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.indexOf("Apple") + 1;

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Syntax

array.indexOf(item, start)

item
Required. The item to search for.

start
Optional. Where to start the search. Negative values will start at the given position counting from the end, and search to the end.

Array.indexOf() returns -1 if the item is not found.
If the item is present more than once, it returns the position of the first
occurrence.

JavaScript Array lastIndexOf()
Array.lastIndexOf() is the same as Array.indexOf(), but
returns the position of the last occurrence of the specified element.

Example
Search an array for the item "Apple":

const fruits = ["Apple", "Orange", "Apple", "Mango"];
let position = fruits.lastIndexOf("Apple") + 1;

Try it Yourself »

Syntax

array.lastIndexOf(item, start)

item
Required. The item to search for

start
Optional. Where to start the search. Negative values will start at the given position counting from the end, and search to the beginning

JavaScript Array find()
The find() method returns the value of the first array element that passes a
test function.
This example finds (returns the value of) the first element that is larger
than 18:

Example

const numbers = [4, 9, 16, 25, 29];
let first =
numbers.find(myFunction);function myFunction(value, index, array) {  return
value > 18;}
Try it Yourself »

Note that the function takes 3 arguments:

The item value
The item index
The array itself

Browser Support
find() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:

Chrome
Edge
Firefox
Safari
Opera

Yes
Yes
Yes
Yes
Yes

find() is not supported in Internet Explorer.

JavaScript Array findIndex()
The findIndex() method returns the index of the first array element that
passes a test function.
This example finds the index of the first element that is larger than 18:

Example

const numbers = [4, 9, 16, 25, 29];
let first =
numbers.findIndex(myFunction);function myFunction(value, index, array) {
return
value > 18;}
Try it Yourself »

Note that the function takes 3 arguments:

The item value
The item index
The array itself

Browser Support
findIndex() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:

Chrome
Edge
Firefox
Safari
Opera

Yes
Yes
Yes
Yes
Yes

findIndex() is not supported in Internet Explorer.

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

Example
Create an Array from a String:

Array.from("ABCDEFG");

Try it Yourself »

Browser Support
from() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:

Chrome
Edge
Firefox
Safari
Opera

Yes
Yes
Yes
Yes
Yes

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.

Example
Create an Array Iterator object, containing the keys of the array:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
const keys = fruits.keys();
for (let x of keys) {
text += x + "<br>";}

Try it Yourself »

Browser Support
keys() is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:

Chrome
Edge
Firefox
Safari
Opera

Yes
Yes
Yes
Yes
Yes

keys() is not supported in Internet Explorer.

JavaScript Array entries()

Example
Create an Array Iterator, and then iterate over the key/value pairs:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
const f = fruits.entries();for (let x of f) {
document.getElementById("demo").innerHTML += x;}

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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).
It is supported in all modern browsers:

Chrome
Edge
Firefox
Safari
Opera

Yes
Yes
Yes
Yes
Yes

entries() is not supported in Internet Explorer.

JavaScript Array includes()
ECMAScript 2016 introduced Array.includes() to arrays.
This allows us to check if an element is present in an array (including NaN, unlike indexOf).

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.includes("Mango"); // is true

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Syntax

array.includes(search-item)

Array.includes() allows to check for NaN values. Unlike Array.indexOf().

Browser Support
includes() is an ECMAScript 2016 feature.
It is supported in all modern browsers:

Chrome
Edge
Firefox
Safari
Opera

Yes
Yes
Yes
Yes
Yes

includes() is not supported in Internet Explorer.

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

Example

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];

Try it Yourself »

Browser Support
... is an ES6 feature (JavaScript 2015).
It is supported in all modern browsers:

Chrome
Edge
Firefox
Safari
Opera

Yes
Yes
Yes
Yes
Yes

... is not supported in Internet Explorer.

Complete Array Reference
For a complete Array reference, go to our:
Complete JavaScript Array Reference.
The reference contains descriptions and examples of all Array
properties and methods.

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Reference: https://www.w3schools.com/js/js_array_iteration.asp