Basics

Arrays

Ordered lists with powerful built-in methods.

1. Explanation

Arrays store ordered collections accessed by index (starting at 0). Useful methods: push/pop (end), shift/unshift (start), slice (copy a range), splice (mutate), map (transform), filter (keep matching), reduce (aggregate), find, includes, indexOf, sort, reverse. map/filter/reduce return new arrays without mutating the original.

2. Example

Try it yourself

const nums = [1, 2, 3, 4, 5];

const doubled = nums.map(n => n * 2);   // [2,4,6,8,10]
const evens   = nums.filter(n => n % 2 === 0); // [2,4]
const sum     = nums.reduce((a, n) => a + n, 0); // 15

console.log(doubled);
console.log(evens);
console.log("sum =", sum);
console.log("includes 3?", nums.includes(3));

Console

Click Run to see output.

3. Expected output

output

[2,4,6,8,10]
[2,4]
sum = 15
includes 3? true

4. Common mistakes

✗Mutating with splice when map/filter would be cleaner
✗Forgetting that sort() converts to strings by default — use (a,b)=>a-b for numbers
✗Reading arr[arr.length] expecting the last element — it's undefined; use arr[arr.length-1]

5. Practice questions

Find the largest number in an array without using Math.max
Remove duplicates from an array using new Set
Reverse an array without using .reverse()

→ Try more practice problems