What is partial application?

Partial application means fixing some of a function's arguments now and getting back a new function that waits for the rest. It turns a general function into a more specific one without rewriting it.

function request(method, url, body) {
  return fetch(url, { method, body });
}

const post = request.bind(null, "POST");
post("/api/users", JSON.stringify({ name: "Mai" }));

bind is the built-in way; a closure does the same thing and reads more clearly when you want control over argument order:

const partial = (fn, ...fixed) => (...rest) => fn(...fixed, ...rest);

const add = (a, b, c) => a + b + c;
const add10 = partial(add, 10);
add10(5, 2);          // 17

Not the same as currying

The two get confused constantly. Currying converts f(a, b, c) into a chain f(a)(b)(c) where every call takes exactly one argument. Partial application fixes any number of arguments in one go and returns a function taking however many remain:

curried(1)(2)(3);     // currying — one at a time, always
partial(fn, 1, 2)(3); // partial application — two now, rest later

Where it is useful

• Configuring a generic helper once: const logError = log.bind(null, "ERROR").
• Pre-binding a dependency so callers do not have to pass it every time: const findUser = query.bind(null, db).
• Building small, readable pipelines in functional code.

Two practical notes: the fixed arguments always come first, so design parameter order with the most stable values leftmost. And remember every bind produces a new function identity — cache it if you plan to remove it as an event listener later.