JavaScript Array Methods Cheat Sheet

Covers core iteration and transformation methods on arrays, including map, filter, reduce, the find/some/every family, and mutating versus non-mutating operations.

1 PageBeginnerMar 25, 2026

map, filter, reduce

The core trio for transforming arrays without mutation.

javascript

const nums = [1, 2, 3, 4, 5];nums.map(n => n * 2);              // [2, 4, 6, 8, 10] -- new array, same lengthnums.filter(n => n % 2 === 0);     // [2, 4] -- new array, matching elements onlynums.reduce((sum, n) => sum + n, 0);        // 15 -- folds to a single valuenums.reduce((acc, n) => {                   // Build an object  acc[n] = n * n;  return acc;}, {});   // {1:1, 2:4, 3:9, 4:16, 5:25}// Chainingnums.filter(n => n % 2 === 0).map(n => n * 10);   // [20, 40]

find, findIndex, some, every, includes

Locate elements or test conditions across an array.

javascript

const users = [  { id: 1, name: "Alice", active: true },  { id: 2, name: "Bob", active: false },];users.find(u => u.id === 2);          // { id: 2, name: "Bob", ... } or undefinedusers.findIndex(u => u.id === 2);     // 1 (index) or -1users.some(u => u.active);            // true -- at least one matchesusers.every(u => u.active);           // false -- not all match[1, 2, 3].includes(2);                // true -- value membership check

Mutating Methods (Modify in Place)

These change the original array and return something else.

javascript

const arr = [3, 1, 2];arr.push(4);          // [3, 1, 2, 4] -- add to end, returns new lengtharr.pop();             // [3, 1, 2] -- remove from end, returns removed itemarr.unshift(0);        // [0, 3, 1, 2] -- add to startarr.shift();           // [3, 1, 2] -- remove from startarr.sort();                    // Mutates! Default sort is lexicographic (string) orderarr.sort((a, b) => a - b);     // Numeric ascending sort -- always pass a comparator for numbersarr.splice(1, 1, "x", "y");    // At index 1, remove 1 item, insert "x","y"arr.reverse();                 // Mutates in place

Non-Mutating Alternatives (ES2023)

Newer methods that leave the original array untouched.

javascript

const arr = [3, 1, 2];arr.toSorted((a, b) => a - b);   // [1, 2, 3] -- arr itself unchangedarr.toReversed();                // New reversed arrayarr.with(0, 99);                 // New array with index 0 replaced by 99// Combine & flatten[1, [2, 3], [4, [5]]].flat();          // [1, 2, 3, 4, [5]] -- flattens 1 level by default[1, [2, 3], [4, [5]]].flat(Infinity);  // [1, 2, 3, 4, 5][1, 2].flatMap(n => [n, n * 10]);   // [1, 10, 2, 20] -- map then flatten 1 level

Quick Reference

Other frequently used array methods.

Array.from({length: 5}, (_, i) => i)- Build an array from an array-like/iterable, with an optional mapper
Array.isArray(x)- Reliable check for array type
arr.join(', ')- Concatenate elements into a string with a separator
arr.slice(1, 3)- Non-mutating: extract a sub-array by index range
arr.indexOf(x)- Find the first index of a primitive value, -1 if absent
arr.at(-1)- Access from the end using negative indices (ES2022)

Pro Tip

Array.prototype.sort() converts elements to strings and sorts lexicographically by default -- [10, 2, 1].sort() gives [1, 10, 2], not numeric order -- so always pass a comparator like (a, b) => a - b for numbers.

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