What is a Higher-Order Function?

Definition

A Higher-Order Function is a function that satisfies at least one of these conditions:

1. Accepts a function as an argument
2. Returns a function

This is a core concept in functional programming. JavaScript supports higher-order functions natively because functions are first-class citizens.

Examples

1. Accepting a Function as an Argument

// map, filter, and reduce are all higher-order functions
const numbers = [1, 2, 3, 4, 5];

const doubled = numbers.map(n => n * 2);       // [2, 4, 6, 8, 10]
const evens = numbers.filter(n => n % 2 === 0); // [2, 4]
const sum = numbers.reduce((acc, n) => acc + n, 0); // 15

// Custom higher-order function
function applyToAll(arr, fn) {
  return arr.map(fn);
}
applyToAll([1, 2, 3], x => x * x); // [1, 4, 9]

2. Returning a Function

// Function factory
function multiplier(factor) {
  return function(number) {
    return number * factor;
  };
}

const double = multiplier(2);
const triple = multiplier(3);

double(5); // 10
triple(5); // 15

3. Currying

const add = a => b => a + b;
const add5 = add(5);
add5(3);  // 8
add5(10); // 15

Benefits

• Reusability: Separates generic logic from specific operations
• Conciseness: Reduces repetitive code
• Composability: Functions can be freely combined
• Testability: Pure functions have no side effects