Arrays in JavaScript

An array is an ordered list. Same indexing rules as most languages: it starts at zero.

const marks = [88, 72, 95, 61];
marks[0];           // 88
marks.length;       // 4
marks.at(-1);       // 61 — last item

The four methods worth learning first

// map — same length, each item transformed
const doubled = marks.map(m => m * 2);        // [176, 144, 190, 122]

// filter — the items that pass a test
const good = marks.filter(m => m >= 80);      // [88, 95]

// find — the first match, or undefined
const first = marks.find(m => m < 70);        // 61

// reduce — everything down to one value
const total = marks.reduce((sum, m) => sum + m, 0);   // 316

Each takes a function and runs it on every item. Once these are comfortable, most for loops in your code disappear, and what is left reads like a description of what you wanted rather than a list of steps.

Which ones change the original

This catches people out, so learn the split:

Return something new, original untouched: map, filter, slice, concat, join, toSorted

Change the array in place: push, pop, shift, unshift, splice, sort, reverse

const original = [3, 1, 2];
const sorted = original.sort();
// original is now [1, 2, 3] too — sort changed it

If that matters, copy first: [...original].sort().

The sort surprise

[10, 9, 100].sort();     // [10, 100, 9]

Not a bug. sort() converts items to strings by default, and "100" sorts before "9". For numbers, pass a comparison:

[10, 9, 100].sort((a, b) => a - b);    // [9, 10, 100]

Copying

const a = [1, 2, 3];
const b = a;          // same array, two names
b.push(4);            // a is [1,2,3,4] as well

const c = [...a];     // a real copy

The spread ... copies one level deep. An array of objects copied this way still shares the objects inside it.