JavaScript Array Methods Cheatsheet

1. Mutator methods

push(): Adds one or more elements to the end of an array and returns the new length of the array.

let arr = [1, 2, 3];
arr.push(4, 5); // returns 5
// arr is now [1, 2, 3, 4, 5]

pop(): Removes the last element from an array and returns that element.

let arr = [1, 2, 3];
let last = arr.pop(); // returns 3
// arr is now [1, 2]

shift(): Removes the first element from an array and returns that element.

let arr = ["a", "b", "c"];
let first = arr.shift(); // returns "a"
// arr is now ["b", "c"]

unshift(): Adds one or more elements to the beginning of an array and returns the new length of the array.

let arr = [1, 2, 3];
arr.unshift(0); // returns 4
// arr is now [0, 1, 2, 3]

splice(): Changes the contents of an array by removing or replacing existing elements and/or adding new elements in place.

let arr = [1, 2, 3, 4, 5];
arr.splice(2, 0, "a", "b"); // returns [], no elements removed
// arr is now [1, 2, "a", "b", 3, 4, 5]

reverse(): Reverses the order of the elements of an array in place.

let arr = [1, 2, 3];
arr.reverse(); 
// arr is now [3, 2, 1]

sort(): Sorts the elements of an array in place and returns the array.

let arr = [1, 3, 2];
arr.sort(); 
// arr is now [1, 2, 3]

concat(): Returns a new array that is this array joined with other array(s) and/or value(s).

let arr = [1, 2, 3];
let newArr = arr.concat([4, 5]); 
// newArr is [1, 2, 3, 4, 5]

join(): Joins all elements of an array into a string.

let arr = ["Hello", "world"];
let str = arr.join(" "); 
// str is "Hello world"

slice(): Extracts a section of the calling array and returns a new array.

let arr = [1, 2, 3, 4, 5];
let newArr = arr.slice(1, 3); 
// newArr is [2, 3]

indexOf(): Returns the first (least) index of an element within the array equal to the specified value, or -1 if none is found.

let arr = [1, 2, 3, 4, 5];
let index = arr.indexOf(3); 
// index is 2

lastIndexOf(): Returns the last (greatest) index of an element within the array equal to the specified value, or -1 if none is found.

let arr = [1, 2, 3, 2, 

1];
let index = arr.lastIndexOf(2); 
// index is 3

forEach(): Executes a provided function once per array element.

let arr = [1, 2, 3];
arr.forEach(item => console.log(item));
// outputs 1, 2, 3

map(): Creates a new array with the results of calling a provided function on every element in this array.

let arr = [1, 2, 3];
let newArr = arr.map(item => item * 2);
// newArr is [2, 4, 6]

filter(): Creates a new array with all elements that pass the test implemented by the provided function.

let arr = [1, 2, 3, 4, 5];
let newArr = arr.filter(item => item > 3);
// newArr is [4, 5]

reduce(): Applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

let arr = [1, 2, 3, 4, 5];
let sum = arr.reduce((total, value) => total + value, 0);
// sum is 15

some(): Tests whether some element in the array passes the test implemented by the provided function.

let arr = [1, 2, 3, 4, 5];
let hasLargeNumber = arr.some(item => item > 4);
// hasLargeNumber is true

every(): Tests whether all elements in the array pass the test implemented by the provided function.

let arr = [1, 2, 3, 4, 5];
let allGreaterThanZero = arr.every(item => item > 0);
// allGreaterThanZero is true

find(): Returns the value of the first element in the array that satisfies the provided testing function.

let arr = [1, 2, 3, 4, 5];
let firstLargeNumber = arr.find(item => item > 3);
// firstLargeNumber is 4

findIndex(): Returns the index of the first element in the array that satisfies the provided testing function.

let arr = [1, 2, 3, 4, 5];
let firstLargeNumberIndex = arr.findIndex(item => item > 3);
// firstLargeNumberIndex is 3

includes(): Determines whether an array includes a certain element, returning true or false as appropriate.

let arr = [1, 2, 3, 4, 5];
let includesThree = arr.includes(3);
// includesThree is true