JavaScript Arrays: Create, Access, Change and Measure

An array is an ordered list of values. It is how you store a to-do list, a set of prices, or the results of a search.

script.js

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

console.log(fruits);         // → ["apple", "banana", "cherry"]
console.log(fruits.length);  // → 3

Square brackets create the array, and items are separated by commas. An array can hold any values, even mixed types, though lists of one kind of value are easier to work with.

Reading items by index

Positions are called indexes, and they start at 0.

script.js

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

console.log(fruits[0]);          // → apple
console.log(fruits[2]);          // → cherry
console.log(fruits[5]);          // → undefined
console.log(fruits.at(-1));      // → cherry   (last item)
console.log(fruits[fruits.length - 1]); // → cherry

at(-1) counts from the end, which avoids the length - 1 calculation.

Changing items

script.js

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

fruits[1] = "blueberry";
console.log(fruits); // → ["apple", "blueberry", "cherry"]

You can change items of a const array. What const prevents is pointing the name at a different array, see let and const.

Length

length is always one more than the last index. Setting it shorter cuts the array.

script.js

1
2
3

const letters = ["a", "b", "c", "d"];
letters.length = 2;
console.log(letters); // → ["a", "b"]

Creating arrays

script.js

const empty = [];
const numbers = [1, 2, 3];
const mixed = ["text", 42, true, null, [1, 2]];
const filled = new Array(3).fill(0);
const range = Array.from({ length: 5 }, (_, i) => i + 1);

console.log(filled); // → [0, 0, 0]
console.log(range);  // → [1, 2, 3, 4, 5]
console.log(mixed.length); // → 5

Arrays are objects

script.js

1
2
3

console.log(typeof []);            // → object
console.log(Array.isArray([]));    // → true
console.log(Array.isArray("abc")); // → false

Use Array.isArray() to check for an array, because typeof cannot tell arrays from other objects.

lightbulb

Arrays hold references

An array of objects holds references to those objects. If two arrays share the same object, changing it in one is visible in the other. See copying objects.