Arrays in JavaScript

An array is an ordered list of values, indexed from zero. JavaScript arrays can hold any mix of types, including other arrays and objects, and they resize automatically as you add or remove items.

Creating and accessing arrays

const fruits = ["apple", "banana", "cherry"];

fruits[0];          // "apple"
fruits[fruits.length - 1]; // "cherry" -- last item
fruits.length;       // 3

Indexing past the end doesn't throw — it just returns undefined:

fruits[10]; // undefined

Adding and removing items

fruits.push("date");     // add to the end -- returns new length
fruits.pop();             // remove from the end -- returns removed item
fruits.unshift("apricot"); // add to the start
fruits.shift();            // remove from the start

push/pop operate on the end of the array and are fast; unshift/shift operate on the start and are slower, since every other item has to be re-indexed.

Checking and finding

fruits.includes("banana"); // true
fruits.indexOf("cherry");   // 2 (-1 if not found)
fruits.find(f => f.startsWith("b")); // "banana" -- first match

Slicing vs splicing

These two are easy to confuse and do very different things. slice copies a portion of an array without changing the original; splice mutates the original in place.

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

nums.slice(1, 3);   // [2, 3] -- new array, nums unchanged

nums.splice(1, 2);  // removes 2 items starting at index 1
console.log(nums);   // [1, 4, 5] -- original array mutated

Arrays are objects, checked with Array.isArray

typeof [];            // "object" -- not helpful for telling arrays apart
Array.isArray([]);    // true
Array.isArray({});    // false

Since typeof reports arrays as plain "object", Array.isArray() is the reliable way to check.

Mutating vs non-mutating methods

Some array methods change the array in place (push, pop, splice, sort, reverse); others return a new array and leave the original untouched (slice, map, filter, concat). This distinction matters a lot once you're passing arrays around — mutating an array another part of the program still holds a reference to can cause bugs that are hard to trace. When in doubt, prefer the non-mutating version.

The next lesson covers objects — JavaScript's other core data structure, for storing key-value data instead of an ordered list.