Data Structure: Arrays in JavaScript

Overview

Arrays are ordered, zero‑indexed lists that can hold values of any type. They are dynamic and preserve insertion order. Arrays are also iterable, making them easy to use with loops and iteration helpers.

const arr = [1, 2, 3, 'four', { five: 5 }];

Creating Arrays

const empty = [];
const numbers = [1, 2, 3];
const mixed = [1, 'two', { three: 3 }];
const fromString = Array.from('abc');
const filled = Array(4).fill(0);

Reading & Iteration

const arr = [1, 2, 3];
for (const item of arr) console.log(item);
arr.forEach((item, i) => console.log(i, item));

Add & Remove at Ends

const arr = [1, 2, 3];
arr.push(4);
arr.unshift(0);

Result:

0
1
2
3
4

Remove from ends:

arr.pop();
arr.shift();
for (const item of arr) console.log(item);

Result:

1
2
3

Common Methods

• map: transform each item into a new array
• filter: keep items that match a condition
• reduce: accumulate values into a single result
• concat: combine arrays
• slice: copy a range (non‑mutating)
• splice: insert/remove inside the array (mutating)

Examples:

const nums = [1, 2, 3, 4];
const doubled = nums.map(n => n * 2);
const evens = nums.filter(n => n % 2 === 0);
const sum = nums.reduce((a, n) => a + n, 0);

Copying Arrays

const original = [1, 2, 3];
const copy1 = original.slice();
const copy2 = [...original];
const copy3 = Array.from(original);

These are shallow copies. Nested objects remain shared.

Sorting

const words = ['b', 'a', 'c'];
words.sort();

const nums = [10, 2, 5];
nums.sort((a, b) => a - b);

sort mutates the array. Provide a comparator for numeric sorts.

Big‑O Complexity

Operation	Complexity
Insert/remove at end (push/pop)	O(1)
Insert/remove at start (shift/unshift)	O(n)
Access by index	O(1)
Search (includes/indexOf/find)	O(n)
concat / slice / splice	O(n)
forEach / map / filter / reduce	O(n)

Notes & Pitfalls

• Arrays are zero‑indexed
• shift/unshift reindex elements and are O(n)
• Prefer non‑mutating methods when working with immutable patterns or UI state