<script> function sum(valueA) { throw 'Not implemented' } console.log(sum(1)()); // 1 console.log(sum(1)(2)()); // 3 console.log(sum(1)(2)(-3)()); // 0 </script>

The sum function should support chaining calls with arguments and end with a call without arguments that returns the sum of all passed numbers. This can be achieved using a closure that accumulates the sum and returns a function that either accepts the next argument or, if called without arguments, returns the result.

Example implementation in JavaScript:

function sum(valueA) {
  let total = valueA;
  function inner(valueB) {
    if (arguments.length === 0) {
      return total;
    }
    total += valueB;
    return inner;
  }
  return inner;
}

console.log(sum(1)());       // 1
console.log(sum(1)(2)());    // 3
console.log(sum(1)(2)(-3)()); // 0