--description--

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.

--hints--

addTogether(2, 3) should return 5.

assert.deepEqual(addTogether(2, 3), 5);

addTogether(23.4, 30) should return 53.4.

assert.deepEqual(addTogether(23.4, 30), 53.4);

addTogether("2", 3) should return undefined.

assert.isUndefined(addTogether('2', 3));

addTogether(5, undefined) should return undefined.

assert.isUndefined(addTogether(5, undefined));

addTogether("https://www.youtube.com/watch?v=dQw4w9WgXcQ") should return undefined.

assert.isUndefined(addTogether('https://www.youtube.com/watch?v=dQw4w9WgXcQ'));

addTogether(5) should return a function.

assert.deepEqual(typeof(addTogether(5)), 'function');

addTogether(5)(7) should return 12.

assert.deepEqual(addTogether(5)(7), 12);

addTogether(2)([3]) should return undefined.

assert.isUndefined(addTogether(2)([3]));

addTogether(2, "3") should return undefined.

assert.isUndefined(addTogether(2, '3'));

--seed--

--seed-contents--

function addTogether() {
  return false;
}

addTogether(2,3);

--solutions--

function addTogether() {
  const first = arguments[0];
  if (typeof(first) !== 'number') {
    return undefined;
  }
  if (arguments.length === 1) {
    return function(second) {
      if (typeof(second) !== 'number') {
        return undefined;
      }
      return first + second;
    };
  }
  const second = arguments[1];
  if (typeof(second) !== 'number') {
    return undefined;
  }
  return first + second;
}