Higher-order functions

Learn higher-order functions that take or return functions.

Higher-order functions

A higher-order function either takes a function as a parameter or returns a function. In JavaScript, map, filter, and reduce are higher-order functions because they accept a callback function.

[1, 2, 3].map(x => x * 2);

Here map takes the function x => x * 2 and applies it to each element. Higher-order functions make code short and reusable; they let you split business logic into small functions.

Functions that return functions

A higher-order function can also return a function. This is like building a factory: the outer function takes a setting, the inner function does the work using that setting.

function createMultiplier(factor) {
  return n => n * factor;
}
const double = createMultiplier(2);

Here calling createMultiplier(2) produces a new function called double. factor is kept in a closure and used on every call. This is a powerful way to build pre-configured functions.

Exercise Write the createMultiplier factory

Write a higher-order function named createMultiplier. It takes a factor parameter and returns a function that computes n => n * factor. Then call const double = createMultiplier(2); and console.log(double(5));.

Show example solution

function createMultiplier(factor) {
  return n => n * factor;
}
const double = createMultiplier(2);
console.log(double(5));

Test yourself

Which one is a higher-order function?
- Math.max(1, 2, 3)
- 'abc'.toUpperCase()
- [1, 2].filter(x => x > 1)
- Number('42')
Show answer

[1, 2].filter(x => x > 1) — Correct! filter takes a function, so it is higher-order.

Call the prefix factory

const withPrefix = prefix => text => prefix + text;
const greet = withPrefix(____);

Show answer

const withPrefix = prefix => text => prefix + text;
const greet = withPrefix('Hello ');

Match the higher-order examples

map

takes a function
withPrefix

returns a function
filter

takes a predicate

Next lesson: Summary - Functional programming

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