Arrays

An array holds an ordered list of values in square brackets: const fruits = ["apple", "banana"]. Access items by their zero-based index (fruits[0] is "apple").

Example: map and filter, the two most-used array methods

const numbers = [1, 2, 3, 4, 5];
const doubled = numbers.map((n) => n * 2);
const evens = numbers.filter((n) => n % 2 === 0);

document.getElementById("output").textContent =
  "Doubled: " + doubled.join(", ") + " | Evens: " + evens.join(", ");

.map() transforms every item into something new and returns a brand-new array of the same length, here, each number becomes double itself. .filter() keeps only the items where the given function returns true, discarding the rest, here, only the even numbers survive. Neither method changes the original numbers array at all; both return a completely new one, which is a deliberate JavaScript convention worth internalizing early, since it avoids a whole category of bugs where code accidentally mutates data something else is still relying on.