Intermediate Algorithm Scripting: Arguments Optional

currying-challenge

Create a function that sums two arguments together. 
If only one argument is provided, then return a function that expects one argument and returns the sum.

For example, addTogether(2, 3) should return 5, and addTogether(2) should return a function.
Calling this returned function with a single argument will then return the sum:
var sumTwoAnd = addTogether(2);
sumTwoAnd(3) returns 5.
If either argument isn't a valid number, return undefined.

Solution: 
function addTogether(...args) {
  const length = args.length; 

  if(length === 1) { 
    if(typeof args[0] !== "number") {
      return undefined; 
    }
    return function(y) {
      if(typeof y !== "number") {
        return undefined; 
      }
      return args[0] + y;
    }
  }
  
  if(typeof args[1] !== "number") {
    return undefined; 
  }
  return args[0] + args[1]; 
}

addTogether(2,3);


Link: 
https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/arguments-optional