JavaScript Array Methods – map(), filter(), reduce() & More

JavaScript arrays come with built-in methods that make data processing easier, faster, and more readable.

1. map() – Transform Each Element

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

Example:

const numbers = [1, 2, 3, 4];
const doubled = numbers.map(num => num * 2);
console.log(doubled);

Output

Use When: You want to change or transform each item.

2. filter() – Keep Some Elements

The filter() method returns a new array with elements that pass a condition.

Example:

const ages = [10, 18, 21, 15];
const adults = ages.filter(age => age >= 18);
console.log(adults);

Output

Use When: You want to remove unwanted items from an array.

3. reduce() – Turn Array into a Single Value

The reduce() method reduces the array to a single value by applying a function on each element.

Example:

const prices = [10, 20, 30];
const total = prices.reduce((sum, price) => sum + price, 0);
console.log(total);

Output

Use When: You need to sum, multiply, or accumulate values.

4. forEach() – Loop Over Elements

The forEach() method executes a function for each item, but doesn’t return anything.

Example:

const fruits = ["apple", "banana", "cherry"];
            
fruits.forEach(fruit => console.log("I like", fruit));
            

Output

Use When: You need a simple loop with side effects (e.g., printing, DOM changes).

5. find() – Get First Match

The find() method returns the first element that matches a condition.

Example:

const users = [{id: 1}, {id: 2}, {id: 3}];
const user = users.find(u => u.id === 2);
console.log(user);

Output

Use When: You want just the first match.

6. some() and every()

some() → Returns true if any element matches

every() → Returns true if all elements match

Example:

const scores = [70, 80, 90];
console.log(scores.some(score => score < 60)); // false
console.log(scores.every(score => score >= 70)); // true

7. includes() – Check if Array Contains Value

Example:

const colors = ["red", "green", "blue"];
console.log(colors.includes("green")); // true
console.log(colors.includes("yellow")); // false

8. sort() – Sort Array Elements

Be careful — sort() modifies the original array.

Example:

const nums = [3, 1, 4, 2];
nums.sort(); // Default sort is alphabetical
console.log(nums); // [1, 2, 3, 4]
nums.sort((a, b) => b - a); // Descending
console.log(nums); // [4, 3, 2, 1]

9. splice() and slice()

• splice() → Modifies array (add/remove)
• slice() → Returns copy of array portion

Example:

const items = ['a', 'b', 'c', 'd'];
const removed = items.splice(1, 2); // Removes 2 items from index 1
console.log(removed); // ['b', 'c']

const copy = items.slice(0, 2);
console.log(copy); // ['a', 'd']

Summary Table: Array Methods