Trying to initialize a function by adding two numbers then transforming the sum into a binary string, what's missing?

Question

function addBinary(a,b) {
  var sum = a + b;
  return sum;

  function decToBin(sum) {
    return (sum >>> 0).toString(2);
  }
}

Answer 1

function addBinary(a,b) {
  var sum = a + b;

  //I added this line!
  //Instead of returning "sum", it returns output of "decToBin"
  return decToBin(sum);

  function decToBin(sum) {
    return (sum >>> 0).toString(2);
  }
}

Answer 2

I think this is what you want.

function addBinary(a,b) {
     var sum = a + b;
     return decToBin(sum);

     function decToBin(sum) {
         return dec2bin(sum);
     }
 }

or a shorter one like this.

function addBinary(a,b) {
    var sum = a + b;
    return dec2bin(sum);
}

You can also have negative numbers without a problem.