In Curry I and II, we relied on the function's length property to know when to stop. In Curry III, the function is variadic (uses rest parameters like ...args), meaning func.length is usually 0. There is no "final" number of arguments.

The Challenge: The function must be infinitely callable: f(1)(2)(3)...(n). But if the function is always returning a new function, how do we ever get the numeric result?

The "Brainteaser" Solution: We must override JavaScript's internal type conversion methods. When the engine expects a primitive (like when you console.log or use an operand), it looks for specific symbols or methods on the object to "coerce" it into a value.

Key Internal Methods to Override:

Symbol.toPrimitive: The modern standard for hint-based coercion.
valueOf(): Fallback for numeric coercion.
toString(): Fallback for string coercion.

💡 Examples

function add(...nums) {
  return nums.reduce((a, b) => a + b, 0);
}

const curried = curry(add);

const addFive = curried(5);
console.log(addFive);       // 5 (Coerced to number)
console.log(addFive(10));   // 15 (Coerced to number)
console.log(addFive(2)(3)); // 10 (Coerced to number)