Lesson 21 — Currying

One argument at a time

Currying turns a function that takes two arguments, f(a, b), into a function that takes one and returns another: f(a)(b). The first call "remembers" a through a closure. That lets you make specialised helpers from a general one.

const add = (a) => (b) => a + b;
const addTen = add(10);
console.log(addTen(5)); // 15

Why it matters: this style is everywhere in functional code and libraries (event handlers, configured validators, Redux middleware).