Currying in JavaScript: Transforming Functions Step by Step

Currying is a technique of transforming a function with multiple arguments into a sequence of functions, each taking one argument.

Why is Currying Needed?

• Reusability: you can fix some arguments in advance.
• Function purity: curried functions are easier to test and combine.
• Functional style: often used in functional programming.

Simple Example

function sum(a) {
  return function (b) {
    return function (c) {
      return a + b + c;
    };
  };
}

console.log(sum(1)(2)(3)); // 6

Here sum is a curried function, it takes one argument at a time.

Partial Application

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

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

Universal Currying Function

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

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

const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)(3)); // 6
console.log(curriedAdd(1, 2)(3)); // 6

Difference from Regular Function

When to Apply Currying?

• When you need to partially apply arguments
• When using functional style (e.g., with map, filter, reduce)
• When composing complex functions from simple ones