Explain currying and partial application in JavaScript

Question

Explain currying and partial application in JavaScript.

Answer

Currying transforms a function with multiple parameters into a sequence of functions that take one parameter at a time.

function multiply(a) {
  return function (b) {
    return a * b;
  };
}

const double = multiply(2);
console.log(double(5)); // 10

Partial application pre-fills some arguments of a function:

function add(a, b, c) {
  return a + b + c;
}

const add5 = add.bind(null, 5);
console.log(add5(10, 2)); // 17

Real-World Example

Used in React/Node to create specialized functions from generic ones — e.g. makeApiClient(baseUrl).

Quick Practice

Write a curried version of a function that adds two numbers: add(a)(b) → returns a + b.

Summary

Currying = break into smaller functions. Partial = pre-fill arguments. Both improve reusability.