Array Methods: map, filter, reduce

Why Use These Methods?

map(), filter(), and reduce() are functional programming methods that replace most manual loops. They make your code more readable, less error-prone, and often shorter.

map() — Transform Every Element

map() creates a new array by applying a function to every element.

let numbers = [1, 2, 3, 4, 5];
let doubled = numbers.map(n => n * 2);
// [2, 4, 6, 8, 10]

filter() — Keep Only Some Elements

filter() creates a new array with only elements that pass a test.

let numbers = [1, 2, 3, 4, 5, 6];
let evens = numbers.filter(n => n % 2 === 0);
// [2, 4, 6]

reduce() — Reduce to a Single Value

reduce() accumulates array elements into a single value.

let numbers = [1, 2, 3, 4, 5];
let sum = numbers.reduce((acc, curr) => acc + curr, 0);
// 15

Common Mistakes

• Using map() when you should use forEach() (or vice versa)
• Forgetting that map(), filter(), and reduce() don't modify the original array
• Not providing an initial value for reduce() when needed

Quick Quiz

Summary

• map(): transform every element → returns a new array
• filter(): keep only matching elements → returns a new array
• reduce(): accumulate to a single value → returns a single value
• These methods don't modify the original array
• Chain methods for powerful data transformations