JavaScript Array Methods

Adds one or more items to the end, returns the new length.

const cart = ['apple'];
cart.push('banana'); // 2

Removes and returns the last item.

const stack = ['init', 'save', 'close'];
stack.pop(); // 'close'

Removes and returns the first item.

const queue = ['first', 'second', 'third'];
queue.shift(); // 'first'

Inserts, removes, or replaces items at any index. Mutates in place.

const list = ['a', 'b', 'c'];
list.splice(1, 1, 'x'); // returns ['b']
// list is now ['a', 'x', 'c']

Adds one or more items to the beginning, returns the new length.

const queue = ['second', 'third'];
queue.unshift('first'); // 3

Merges two or more arrays into a new array without mutation.

const arr1 = [1, 2];
arr1.concat([3, 4]); // [1, 2, 3, 4]

Copies or combines arrays cleanly without mutation.

const a = [1, 2];
const b = [...a, 3];               // [1, 2, 3]
const merged = [...a, ...b, ...c]; // all combined

Copies a whole array or a portion without mutating the source.

const arr = [1, 2, 3, 4];
arr.slice(1, 3); // [2, 3]

Builds a new array from arguments, even a single number.

Array.of(7); // [7]
Array(7);    // empty array with length 7

Deep-copies an array, including nested objects, with no shared references.

const original = [{id: 1}];
const copy = structuredClone(original);

Creates a new array by transforming every element with a function.

const prices = [10, 20, 30];
prices.map(p => p * 1.1); // [11, 22, 33]

Returns a new array keeping only items that pass the test.

const users = [{active: true}, {active: false}];
users.filter(u => u.active); // [{active: true}]

Combines all items into a single value; always pass an initial value.

const nums = [1, 2, 3];
nums.reduce((acc, n) => acc + n, 0); // 6

Returns a new sorted array; the original is left unchanged.

const nums = [3, 1, 2];
nums.toSorted(); // [1, 2, 3], nums unchanged

Returns a new reversed array; the original is left unchanged.

const nums = [1, 2, 3];
nums.toReversed(); // [3, 2, 1], nums unchanged

Returns the first element matching a condition, or undefined.

users.find(u => u.id === 1); // {id: 1, ...}

Returns true if the exact value exists in the array.

['read', 'write'].includes('write'); // true

Returns true if at least one element passes the test.

users.some(u => u.isAdmin); // true or false

Returns true only if all elements pass the test.

tasks.every(t => t.done); // true or false

Returns the index of the first element matching a condition, or -1.

users.findIndex(u => u.id === 1); // 0

Sorts the array in place; always pass a comparator for reliable results.

names.sort((a, b) => a.localeCompare(b));

Reverses the array in place; copy first if you need the original order.

const nums = [1, 2, 3];
nums.reverse(); // [3, 2, 1], mutates nums

Combines all elements into a string with a separator.

['a', 'b', 'c'].join(','); // "a,b,c"

Runs a function on each item for side effects; returns undefined.

items.forEach(item => console.log(item));

Converts any iterable — string, Set, NodeList — into a real array.

Array.from('hi');             // ['h', 'i']
Array.from(new Set([1,1,2])); // [1, 2]

Collapses nested arrays one level deep; pass a depth to go deeper.

const nested = [1, [2, [3]]];
nested.flat();   // [1, 2, [3]]
nested.flat(2);  // [1, 2, 3]

Maps over items and flattens the result one level in a single pass.

const words = ['hi there', 'foo bar'];
words.flatMap(s => s.split(' '));
// ['hi', 'there', 'foo', 'bar']

Accesses an element by index; negative values count from the end.

const arr = [10, 20, 30, 40];
arr.at(0);  // 10
arr.at(-1); // 40