JavaScript 2025 cheatsheet

JavaScript 2025: Arrays

13 common JavaScript 2025 tasks.

Transform every item

Create a new array by applying a function to each item.

const doubled = numbers.map((n) => n * 2);

Create a new array with the items that pass a test.

const adults = users.filter((u) => u.age >= 18);

Add up a property across an array. The second argument is the starting value.

const total = items.reduce((sum, item) => sum + item.price, 0);

Return the first item that passes a test, or undefined if none does.

const user = users.find((u) => u.id === 42);

Check whether any item, or every item, passes a test.

const hasAdmin = users.some((u) => u.role === "admin");
const allActive = users.every((u) => u.active);

Return true if the value is in the array.

const found = [1, 2, 3].includes(2);

Collect the tags of every post into a single array.

const tags = posts.flatMap((post) => post.tags);

Sort ascending. sort() compares as text by default and changes the array, so toSorted with a comparator is safer.

const sorted = numbers.toSorted((a, b) => a - b);

Make a shallow copy with spread.

const copy = [...items];

Use a negative index with at().

const last = items.at(-1);

Build an array of the numbers 0 to 4.

const range = Array.from({ length: 5 }, (_, i) => i);

Convert to a Set and back to an array.

const unique = [...new Set(items)];

Return an object whose keys are the group names. Available in current browsers and Node.js 21 and later.

const byRole = Object.groupBy(users, (user) => user.role);