JavaScript Array Methods: map, filter, reduce, find with Real Examples

JavaScript Array Methods

Array methods like map, filter, and reduce are the backbone of modern JavaScript. They replace verbose for loops with declarative, readable one-liners. Essential for every JS interview.

map() — Transform Every Element

const numbers = [1, 2, 3, 4, 5];

// Double every number
const doubled = numbers.map(n => n * 2);
// [2, 4, 6, 8, 10]

// Extract property from objects
const users = [{id:1,name:'Rahul'},{id:2,name:'Priya'}];
const names = users.map(u => u.name);
// ['Rahul', 'Priya']

// Transform with index
const indexed = numbers.map((n, i) => `${i}: ${n}`);
// ['0: 1', '1: 2', ...]

filter() — Keep Matching Elements

const numbers = [1, 2, 3, 4, 5, 6];

// Keep evens
const evens = numbers.filter(n => n % 2 === 0);
// [2, 4, 6]

// Filter objects
const users = [
    {name:'Rahul', active:true},
    {name:'Priya', active:false},
    {name:'Amit',  active:true},
];
const activeUsers = users.filter(u => u.active);
// [{name:'Rahul',...}, {name:'Amit',...}]

reduce() — Collapse to Single Value

const numbers = [1, 2, 3, 4, 5];

// Sum
const sum = numbers.reduce((acc, n) => acc + n, 0); // 15

// Build object from array
const users = [{id:1,name:'Rahul'},{id:2,name:'Priya'}];
const byId  = users.reduce((acc, u) => {
    acc[u.id] = u;
    return acc;
}, {});
// {1: {id:1,name:'Rahul'}, 2: {id:2,name:'Priya'}}

// Flatten nested arrays
const nested = [[1,2],[3,4],[5]];
const flat   = nested.reduce((acc, arr) => acc.concat(arr), []);
// [1, 2, 3, 4, 5]  — or just use nested.flat()

find(), findIndex(), every(), some()

const users = [{id:1,name:'Rahul',age:25},{id:2,name:'Priya',age:30},{id:3,name:'Amit',age:22}];

// find — returns first match (or undefined)
const priya = users.find(u => u.name === 'Priya');
// {id:2, name:'Priya', age:30}

// findIndex — returns index (or -1)
const idx = users.findIndex(u => u.age < 25);
// 2 (Amit at index 2)

// every — true if ALL match
const allAdults = users.every(u => u.age >= 18); // true

// some — true if ANY match
const hasMinor = users.some(u => u.age < 18); // false

// includes — check value exists
[1,2,3].includes(2); // true

flat() and flatMap()

// flat — flatten nested arrays
const nested = [1, [2, 3], [4, [5, 6]]];
nested.flat();    // [1, 2, 3, 4, [5, 6]]
nested.flat(2);   // [1, 2, 3, 4, 5, 6]

// flatMap — map then flatten one level
const sentences = ['Hello World', 'Foo Bar'];
const words     = sentences.flatMap(s => s.split(' '));
// ['Hello', 'World', 'Foo', 'Bar']