Basic Array Methods

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let size = fruits.length;

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();

Result:

Examples

Get the third element of fruits using at():

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits.at(2);

Get the third element of fruits using []:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits[2];

Note

Many languages allows negative bracket indexing like [-1] to access elements from the end of an object / array / string.

This is not possible in JavaScript, because [] is used for accessing both arrays and objects. obj[-1] refers to the value of key -1, not to the last property of the object.

The at() method was introduced in ES2022 to solve this problem.

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.join(" * ");

Result:

Example

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

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits.pop();

Example

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

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let length = fruits.push("Kiwi");

Example

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

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
let fruit = fruits.shift();

Example

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

Example

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

Array indexes start with 0:

[0] is the first array element
[1] is the second
[2] is the third ...

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[0] = "Kiwi";

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits[fruits.length] = "Kiwi";

Warning !

Using delete() leaves undefined holes in the array.

Use pop() or shift() instead.

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
delete fruits[0];

Example (Merging Two Arrays)

const myGirls = ["Cecilie", "Lone"];
const myBoys = ["Emil", "Tobias", "Linus"];

const myChildren = myGirls.concat(myBoys);

Note

The concat() method does not change the existing arrays. It always returns a new array.

The concat() method can take any number of array arguments.

Example (Merging Three Arrays)

const arr1 = ["Cecilie", "Lone"];
const arr2 = ["Emil", "Tobias", "Linus"];
const arr3 = ["Robin", "Morgan"];
const myChildren = arr1.concat(arr2, arr3);

Example (Merging an Array with Values)

const arr1 = ["Emil", "Tobias", "Linus"];
const myChildren = arr1.concat("Peter"); 

Examples

Copy to index 2, all elements from index 0:

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.copyWithin(2, 0);

Copy to index 2, the elements from index 0 to 2:

const fruits = ["Banana", "Orange", "Apple", "Mango", "Kiwi"];
fruits.copyWithin(2, 0, 2);

Note

The copyWithin() method overwrites the existing values.

The copyWithin() method does not add items to the array.

The copyWithin() method does not change the length of the array.

Example

const myArr = [[1,2],[3,4],[5,6]];
const newArr = myArr.flat();

Example

const myArr = [1, 2, 3, 4, 5, 6];
const newArr = myArr.flatMap(x => [x, x * 10]);

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2, 0, "Lemon", "Kiwi");

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2, 2, "Lemon", "Kiwi");

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(0, 1);

Example

const months = ["Jan", "Feb", "Mar", "Apr"];
const spliced = months.toSpliced(0, 1);

Example

Slice out a part of an array starting from array element 1 ("Orange"):

const fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
const citrus = fruits.slice(1);

Note

The slice() method creates a new array.

The slice() method does not remove any elements from the source array.

Example

Slice out a part of an array starting from array element 3 ("Apple"):

const fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
const citrus = fruits.slice(3);

Example

const fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
const citrus = fruits.slice(1, 3);

Example

const fruits = ["Banana", "Orange", "Lemon", "Apple", "Mango"];
const citrus = fruits.slice(2);

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits.toString();

Example

const fruits = ["Banana", "Orange", "Apple", "Mango"];
document.getElementById("demo").innerHTML = fruits;

Note

All JavaScript objects have a toString() method.

JavaScript Array Search

JavaScript Array indexOf()

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

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

Syntax

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.

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

Syntax

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).

const fruits = ["Banana", "Orange", "Apple", "Mango"];

fruits.includes("Mango"); // is true

Syntax

Browser Support

includes() is an ECMAScript 2016 feature.

ES 2016 is fully supported in all modern browsers since March 2017:

includes() is not supported in Internet Explorer.

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:

const numbers = [4, 9, 16, 25, 29];
let first = numbers.find(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

Browser Support

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

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

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:

const numbers = [4, 9, 16, 25, 29];
let first = numbers.findIndex(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

Browser Support

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

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

findIndex() is not supported in Internet Explorer.

JavaScript Array findLast() Method

ES2023 added the findLast() method that will start from the end of an array and return the value of the first element that satisfies a condition.

const temp = [27, 28, 30, 40, 42, 35, 30];
let high = temp.findLast(x => x > 40);

Browser Support

findLast() is an ES2023 feature.

It is supported in all modern browsers since July 2023:

JavaScript Array findLastIndex() Method

The findLastIndex() method finds the index of the last element that satisfies a condition.

const temp = [27, 28, 30, 40, 42, 35, 30];
let pos = temp.findLastIndex(x => x > 40);

Browser Support

findLastIndex() is an ES2023 feature.

It is supported in all modern browsers since July 2023:

JavaScript Sorting Arrays

Sorting an Array

The sort() method sorts an array alphabetically:

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

Reversing an Array

The reverse() method reverses the elements in an array:

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

By combining sort() and reverse(), you can sort an array in descending order:

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

JavaScript Array toSorted() Method

ES2023 added the toSorted() method as a safe way to sort an array without altering the original array.

The difference between toSorted() and sort() is that the first method creates a new array, keeping the original array unchanged, while the last method alters the original array.

const months = ["Jan", "Feb", "Mar", "Apr"];
const sorted = months.toSorted();

JavaScript Array toReversed() Method

ES2023 added the toReversed() method as a safe way to reverse an array without altering the original array.

The difference between toReversed() and reverse() is that the first method creates a new array, keeping the original array unchanged, while the last method alters the original array.

const months = ["Jan", "Feb", "Mar", "Apr"];
const reversed = months.toReversed();

Numeric Sort

By default, the sort() function sorts values as strings.

This works well for strings ("Apple" comes before "Banana").

If numbers are sorted as strings, "25" is bigger than "100", because "2" is bigger than "1".

Because of this, the sort() method will produce incorrect result when sorting numbers.

You can fix this by providing a compare function:

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a - b});

Use the same trick to sort an array descending:

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b - a});

The Compare Function

The purpose of the compare function is to define an alternative sort order.

The compare function should return a negative, zero, or positive value, depending on the arguments:

When the sort() function compares two values, it sends the values to the compare function, and sorts the values according to the returned (negative, zero, positive) value.

If the result is negative, a is sorted before b.

If the result is positive, b is sorted before a.

If the result is 0, no changes are done with the sort order of the two values.

Example:

The compare function compares all the values in the array, two values at a time (a, b).

When comparing 40 and 100, the sort() method calls the compare function(40, 100).

The function calculates 40 - 100 (a - b), and since the result is negative (-60),  the sort function will sort 40 as a value lower than 100.

You can use this code snippet to experiment with numerically and alphabetically sorting:

<button onclick="myFunction1()">Sort Alphabetically</button>
<button onclick="myFunction2()">Sort Numerically</button>

<p id="demo"></p>

<script>
const points = [40, 100, 1, 5, 25, 10];
document.getElementById("demo").innerHTML = points;

function myFunction1() {
  points.sort();
  document.getElementById("demo").innerHTML = points;
}

function myFunction2() {
  points.sort(function(a, b){return a - b});
  document.getElementById("demo").innerHTML = points;
}
</script>

Sorting an Array in Random Order

Using a sort function, like explained above, you can sort an numeric array in random order

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(){return 0.5 - Math.random()});

The Fisher Yates Method

The points.sort() method in the example above is not accurate. It will favor some numbers over others.

The most popular correct method, is called the Fisher Yates shuffle, and was introduced in data science as early as 1938!

In JavaScript the method can be translated to this:

const points = [40, 100, 1, 5, 25, 10];

for (let i = points.length -1; i > 0; i--) {
  let j = Math.floor(Math.random() * (i+1));
  let k = points[i];
  points[i] = points[j];
  points[j] = k;
}

Find the Lowest (or Highest) Array Value

There are no built-in functions for finding the max or min value in an array.

To find the lowest or highest value you have 3 options:

• Sort the array and read the first or last element
• Use Math.min() or Math.max()
• Write a home made function

Find Min or Max with sort()

After you have sorted an array, you can use the index to obtain the highest and lowest values.

Sort Ascending:

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return a - b});
// now points[0] contains the lowest value
// and points[points.length-1] contains the highest value

Sort Descending:

const points = [40, 100, 1, 5, 25, 10];
points.sort(function(a, b){return b - a});
// now points[0] contains the highest value
// and points[points.length-1] contains the lowest value

Using Math.min() on an Array

You can use Math.min.apply to find the lowest number in an array:

function myArrayMin(arr) {
  return Math.min.apply(null, arr);
}

Math.min.apply(null, [1, 2, 3]) is equivalent to Math.min(1, 2, 3).

Using Math.max() on an Array

You can use Math.max.apply to find the highest number in an array:

function myArrayMax(arr) {
  return Math.max.apply(null, arr);
}

Math.max.apply(null, [1, 2, 3]) is equivalent to Math.max(1, 2, 3).

JavaScript Array Minimum Method

There is no built-in function for finding the lowest value in a JavaScript array.

The fastest code to find the lowest number is to use a home made method.

This function loops through an array comparing each value with the lowest value found:

function myArrayMin(arr) {
  let len = arr.length;
  let min = Infinity;
  while (len--) {
    if (arr[len] < min) {
      min = arr[len];
    }
  }
  return min;
}

JavaScript Array Maximum Method

There is no built-in function for finding the highest value in a JavaScript array.

The fastest code to find the highest number is to use a home made method.

This function loops through an array comparing each value with the highest value found:

function myArrayMax(arr) {
  let len = arr.length;
  let max = -Infinity;
  while (len--) {
    if (arr[len] > max) {
      max = arr[len];
    }
  }
  return max;
}

Sorting Object Arrays

JavaScript arrays often contain objects:

const cars = [
  {type:"Volvo", year:2016},
  {type:"Saab", year:2001},
  {type:"BMW", year:2010}
];

Even if objects have properties of different data types, the sort() method can be used to sort the array.

The solution is to write a compare function to compare the property values:

cars.sort(function(a, b){return a.year - b.year});

Comparing string properties is a little more complex:

cars.sort(function(a, b){
  let x = a.type.toLowerCase();
  let y = b.type.toLowerCase();
  if (x < y) {return -1;}
  if (x > y) {return 1;}
  return 0;
});

Stable Array sort()

ES2019 revised the Array sort() method.

Before 2019, the specification allowed unstable sorting algorithms such as QuickSort.

After ES2019, browsers must use a stable sorting algorithm:

When sorting elements on a value, the elements must keep their relative position to other elements with the same value.

const myArr = [
  {name:"X00",price:100 },
  {name:"X01",price:100 },
  {name:"X02",price:100 },
  {name:"X03",price:100 },
  {name:"X04",price:110 },
  {name:"X05",price:110 },
  {name:"X06",price:110 },
  {name:"X07",price:110 }
];

In the example above, when sorting on price, the result is not allowed to come out with the names in an other relative position like this: