23.create-a-sum.md

23.create-a-sum.md

Problem

https://bigfrontend.dev/problem/create-a-sum

Problem Description

Create a sum(), which makes following possible

const sum1 = sum(1);
sum1(2) == 3; // true
sum1(3) == 4; // true
sum(1)(2)(3) == 6; // true
sum(5)(-1)(2) == 6; // true

Solution

/**
 * @param {number} num
 */
function sum(num) {
  // Declare a function which will be returned by
  // the function sum(), so we can take a new number.
  const func = function (newNum) {
    // Add the new number to the previous one, call sum()
    // and pass the result as argument, so we can take a
    // new number again.
    return sum(num + newNum);
  };

  // Override func's native valueOf function with our custom one that
  // returns the current num, so we can do 'sum(1)(2)(3) == 6'.
  // When encountering an object where a primitive value is expected,
  // JavaScript calls valueOf method to convert the object (in js, functions are objects )
  // to a primitive value.
  // Besides valueOf, JavaScript also has toString to do such 'type coercion'.
  // sum(1)(2)(3) === 6 will result in false, because '===' does not perform
  // type conversion.
  func.valueOf = function () {
    return num;
  };

  return func;
}

Reference

Problem Discuss