When would you use map vs forEach vs reduce vs filter?

• map — transform each element, returns new array of same length
• filter — keep matching elements, returns smaller array
• reduce — accumulate into single value (any type)
• forEach — side effects only, returns undefined, not chainable

const nums = [1,2,3,4,5];
nums.map(n => n * 2);                // [2,4,6,8,10]
nums.filter(n => n % 2 === 0);       // [2,4]
nums.reduce((sum, n) => sum + n, 0); // 15

// Chaining
nums.filter(n => n > 2).map(n => n ** 2); // [9,16,25]