Arrays in JavaScript

Arrays are ordered, mutable collections with zero‑based indexing.

Creating arrays

const empty = []
const nums = [1,2,3]
const fromCtor = Array(3).fill(0)
const ofVals = Array.of(1,2,3)

Indexing and length

nums[0]         // 1
nums[nums.length - 1]

Adding/removing

const xs = [1,2]
xs.push(3)       // add to end
xs.pop()         // remove last
xs.unshift(0)    // add to start
xs.shift()       // remove first

Transform and query

Non‑mutating vs mutating methods; prefer non‑mutating when possible.

[1,2,3].map(n => n * 2)           // [2,4,6]
[1,2,3,4].filter(n => n % 2 === 0)
[1,2,3].reduce((a,b) => a+b, 0)
[1,2,2,3].includes(2)

Slicing and splicing

const a = [10,20,30,40]
a.slice(1, 3)        // [20,30] (non‑mutating)
a.splice(1, 2, 99)   // mutates a

Copy and spread

const copy = [...a]
const merged = [...a, ...[50,60]]

Sorting and comparing

sort mutates and compares as strings by default; supply a comparator for numeric sorts.

[3,10,2].sort((a,b) => a-b)

Iteration

Prefer for..of or array methods; avoid for..in for arrays.

for (const v of nums) {}
nums.forEach(v => {})

Flat and flatMap

[[1,2],[3]].flat()        // [1,2,3]
[1,2,3].flatMap(n => [n, n+10])

Performance notes

• Push/pop are fast; shift/unshift are O(n)
• Avoid creating large holes (sparse arrays) unless needed

Summary

• Use non‑mutating methods for clarity; know when splice/sort mutate
• Spread, slice, and flat simplify copies and reshaping