Arrays: methods every JavaScript developer must know

Arrays Are Ordered Lists

Arrays are objects with integer keys and a length property. The single most important thing to know about array methods: some mutate the original, some return a new array. Mixing them up silently corrupts data.

Mutating Methods — Change the Original

const arr = [1, 2, 3];
arr.push(4);          // [1,2,3,4] — adds to end
arr.pop();            // [1,2,3]   — removes from end
arr.unshift(0);       // [0,1,2,3] — adds to start
arr.shift();          // [1,2,3]   — removes from start
arr.splice(1, 1, 99); // [1,99,3]  — remove and optionally insert
arr.sort((a,b) => a - b); // sorts in place
arr.reverse();        // reverses in place

Non-Mutating Methods — Return New Value

const nums = [3, 1, 4, 1, 5];
nums.slice(1, 3)              // [1, 4] — does not modify nums
nums.concat([9, 10])          // [3,1,4,1,5,9,10]
[[1],[2],[3]].flat()          // [1, 2, 3]

Search Methods

nums.indexOf(4)           // 2
nums.includes(5)          // true
nums.find(n => n > 3)     // 4 — first match value
nums.findIndex(n => n > 3)// 2 — first match index
nums.every(n => n > 0)    // true
nums.some(n => n > 4)     // true

Utilities

Array.from("hi")          // ["h","i"]
Array.isArray([])         // true