What are higher-order functions in JavaScript?

A higher-order function (HOF) is a function that takes another function as an argument, returns a function, or both.

JavaScript treats functions as first-class citizens, so HOFs are everywhere:

// Accepts a callback
const nums = [1, 2, 3]
nums.map(n => n * 2)       // [2, 4, 6]
nums.filter(n => n > 1)    // [2, 3]
nums.reduce((a, b) => a + b, 0) // 6

// Returns a function
function multiplyBy(factor) {
  return (n) => n * factor
}
const double = multiplyBy(2)

Built-in HOFs: map, filter, reduce, sort, forEach, some, every, addEventListener, Promise.then.

Why interviewers ask: HOFs enable abstraction, reuse, and composable pipelines without duplicating loop logic.