TopolearnArrays are the everyday ordered collection for lists of anything.
An array is an ordered list of values of any types: const items = ["a", 2, { id: 3 }]. Indexes start at 0; items.length is the count, and items.at(-1) gets the last element.
Arrays are mutable objects. The core methods for adding and removing:
push(x)/pop()— add/remove at the end (fast).unshift(x)/shift()— add/remove at the start (shifts every element).splice(start, deleteCount, ...newItems)— remove and/or insert in the middle; mutates and returns removed items.slice(start, end)— returns a copy of a portion, without mutating.
Searching: includes(x) returns a boolean, indexOf(x) returns the index or -1. Both compare by value for primitives but by reference for objects, so [{ id: 1 }].includes({ id: 1 }) is false. You'll use find with a callback for that in the array-methods lesson.
concat and join(sep) combine arrays and turn them into strings; Array.from({ length: 3 }, (_, i) => i) builds [0, 1, 2].
Because arrays are objects, assigning one to another variable copies the reference, not the elements: changing b after const b = a changes a too. Make a shallow copy with [...a] or a.slice().
typeof [] is "object"; use Array.isArray() to check. Assigning beyond the end (arr[10] = x on a short array) creates empty "holes" — avoid it. Arrays can be nested to make grids: grid[row][col].
Example
const queue = ["ann", "bob"];
queue.push("cy"); // [ 'ann', 'bob', 'cy' ]
const first = queue.shift(); // "ann"
console.log(first, queue); // ann [ 'bob', 'cy' ]
const letters = ["a", "b", "c", "d", "e"];
console.log(letters.slice(1, 3)); // [ 'b', 'c' ] (no mutation)
const removed = letters.splice(1, 2, "X");
console.log(removed, letters); // [ 'b', 'c' ] [ 'a', 'X', 'd', 'e' ]
console.log(letters.includes("d"), letters.indexOf("z")); // true -1
console.log(letters.at(-1)); // "e"
const a = [1, 2];
const b = a; // same array
const c = [...a]; // shallow copy
b.push(3);
console.log(a, c); // [ 1, 2, 3 ] [ 1, 2 ]Remember
- Indexes start at 0;
at(-1)reads from the end slicecopies,splicemutates — easy to mix up- Assigning an array copies the reference; copy with
[...arr]
Watch out for
- Expecting
includesorindexOfto find an object by its contents — they compare references.