Iteration Methods (forEach, map, filter, reduce, reduceRight, some, every, find, findIndex, findLast, findLastIndex)

Iteration methods traverse the array and execute a function for each element. They can produce a new array, a single value, or a boolean.

Common Iteration Methods

Method	Description
forEach()	Executes a function for each element.
map()	Creates a new array with the results of calling a function on every element.
filter()	Creates a new array with elements that pass a test.
reduce()	Reduces the array to a single value by accumulating left to right.
reduceRight()	Same as reduce but from right to left.
some()	Returns true if at least one element passes the test.
every()	Returns true if all elements pass the test.
find()	Returns the first element that passes the test.
findIndex()	Returns the index of the first element that passes the test.
findLast()	Returns the last element that passes the test (ES2023).
findLastIndex()	Returns the index of the last element that passes the test (ES2023).

Code Example

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

// forEach
numbers.forEach((n) => console.log(n * 2)); // 2,4,6,8,10

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

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

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

// some/every
console.log(numbers.some((n) => n > 4)); // true
console.log(numbers.every((n) => n > 0)); // true

// find/findIndex
const firstLarge = numbers.find((n) => n > 3); // 4
const indexLarge = numbers.findIndex((n) => n > 3); // 3

// findLast/findLastIndex (ES2023)
const lastLarge = numbers.findLast((n) => n > 3); // 5
const lastIndex = numbers.findLastIndex((n) => n > 3); // 4