How Currying Relies on Closures in JavaScript

How Currying Relies on Closures in JavaScript

Currying would not work without closures. Here is how they work together.

The Connection

Each curried function returns a new function that closes over the previous arguments:

function add(a) {
  return function (b) {
    return function (c) {
      // a, b, c are all accessible here via closures
      return a + b + c;
    };
  };
}

The innermost function closes over a and b. Without closures, the inner function would not have access to the outer arguments.

In the Generic curry Function

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) return fn(...args);
    return (...next) => curried(...args, ...next); // closes over args
  };
}

The returned arrow function closes over args and fn. Each partial call accumulates arguments via the closure.

Why Closures Are Essential

• Accumulation: each returned function remembers previous arguments.
• State: the accumulated arguments are the state, kept alive by the closure.
• Evaluation: only when enough arguments are collected does the original function execute.

The Takeaway

Currying relies on closures: each returned function closes over the accumulated arguments. Without closures, the inner functions would not have access to outer arguments. Closures provide the state (accumulated args) and the delayed evaluation (only call the original when enough args are collected).