Array methods

Array methods

• .map(fn) makes a new array by transforming each item.
• .filter(fn) makes a new array of only the items where fn returns true.
• .forEach(fn) runs fn for each item, making no new array.

let nums = [1, 2, 3];
let doubled = nums.map((n) => n * 2);
console.log(doubled);   // → [2,4,6]

Filtering keeps some items

• .filter keeps each item for which the test returns true.

let nums = [3, 7, 2, 9];
let big = nums.filter((n) => n > 4);
console.log(big);   // → [7,9]

Watch out

• .map and .filter return a new array — the original is untouched.
• You hand each one an arrow function that says what to do with one item.

Common mistakes

• map and filter return a NEW array — they do not change the original.
• forEach just loops and returns nothing.

Now you try

• Use .map, .filter or .forEach.
• Press Check answer to test it.