Composition

Function Composition is the process of combining multiple functions together so that the output of one function becomes the input of the next.

In simple words: Function composition lets you build complex operations by chaining small, reusable functions together.

Example Without Composition:

function double(x) {
  return x * 2;
}

function square(x) {
  return x * x;
}

const number = 5;
const result = square(double(number));
console.log(result); // Output: 100

This works fine, but when you have many functions, nesting them gets hard to read. Example With Composition:

function compose(f, g) {
  return function (x) {
    return f(g(x));
  };
}

const doubleThenSquare = compose(square, double);

console.log(doubleThenSquare(5)); // Output: 100

Composing Multiple Functions:

function compose(...functions) {
  return function (value) {
    return functions.reduceRight((acc, fn) => fn(acc), value);
  };
}

const add5 = (x) => x + 5;
const double = (x) => x * 2;
const square = (x) => x * x;

const composed = compose(square, double, add5);

console.log(composed(2)); // Output: 196

1. add5(2) → 7
2. double(7) → 14
3. square(14) → 196

So compose(square, double, add5) runs right-to-left.

Composition vs Currying

Concept	Description	Example
Currying	Breaks one multi-argument function into many single-argument functions	f(a, b) → f(a)(b)
Composition	Combines multiple single-argument functions into one	f(g(h(x)))