How the sum(1)(2)(3) Function Uses Closures

How the sum(1)(2)(3) Function Uses Closures

The curried sum function relies on closures to accumulate the total.

The Code

function sum(a) {
  return function (b) {
    if (b !== undefined) return sum(a + b);
    return a;
  };
}

Closure Analysis

1. sum(1) is called. a = 1. Returns a function that closes over a.
2. The returned function is called with 2. b = 2. b !== undefined, so it returns sum(1 + 2) = sum(3).
3. sum(3) is called. a = 3. Returns a new function that closes over a = 3.
4. The returned function is called with 3. b = 3. Returns sum(3 + 3) = sum(6).
5. sum(6) is called. a = 6. Returns a function.
6. Called with no argument. b = undefined. Returns a = 6.

Each Call Creates a New Closure

Each call to sum creates a new function with a new closure over a new a. The previous closure is replaced by the new one, but the accumulated value is passed forward.

The Takeaway

The curried sum uses closures: each sum(a) call creates a function that closes over a. When called with b, it returns sum(a + b), creating a new closure with the accumulated total. When called with no argument, it returns the accumulated a.