What are Higher-Order Functions? | JavaScript Interview Questions

What You'll Learn

What higher-order functions are
Functions as arguments
Functions returning functions

Definition

A higher-order function is a function that:

Takes one or more functions as arguments, OR
Returns a function

Examples

code.jsJavaScript

// Function as argument
const numbers = [1, 2, 3, 4];
const doubled = numbers.map(n => n * 2);

// Custom higher-order function
function withLogging(fn) {
  return function(...args) {
    console.log('Calling with:', args);
    const result = fn(...args);
    console.log('Result:', result);
    return result;
  };
}

const add = (a, b) => a + b;
const loggedAdd = withLogging(add);
loggedAdd(2, 3);
// Calling with: [2, 3]
// Result: 5

Common Higher-Order Functions

code.jsJavaScript

// Array methods
[1,2,3].map(fn)
[1,2,3].filter(fn)
[1,2,3].reduce(fn, init)
[1,2,3].forEach(fn)
[1,2,3].find(fn)
[1,2,3].some(fn)
[1,2,3].every(fn)

// setTimeout/setInterval
setTimeout(fn, delay)

// Event listeners
element.addEventListener('click', fn)