evaluating javascript base10 to base2 conversion function

I wrote a simple function to turn a number that is in base 10 into a binary number. I've looked up this how to do this on this website and I haven't understood a good deal of it. I'm sure the function I've written is very strange but it seems to do the trick.

Here is a jsfiddle showing it in action: http://ift.tt/2sxs94u

And here is the function code:

var toBinary = function(x) {

  x = parseInt(x);

  if (x < 0) {
    var k = '-' 
    x = Math.sqrt(Math.pow(x, 2));    
  }

  if (x == 0) {
    return 0;
  }

  var y = x;
  var g = [];

  while (Math.log2(y) >= 0) {
    var n = parseInt(Math.log2(y)); 
    g.push(n+1);
    y = y - Math.pow(2, n);
  }

  var binArray = [];
  for (var i = 0; i < g[0]; i++) {
    binArray.push(0);
  }
  for (var i = 0; i < g.length; i++) {
    binArray[binArray.length-g[i]] = 1;
  }

  var binary = binArray.join("");
  return (k || "") + binary;

};

I don't have a lot of tools yet for evaluating this so my question is as follows:

What this function returns looks right, but how can I make sure what I'm getting is actually a binary number and not just a string that looks like a binary number. Am I missing a few key pieces? Or is my approach a dead end?

An additional question: Is there a built-in function in JS to do this for me? Mostly, this is a learning exercise for me so I'm still interested in producing a base10 to base2 number converting function.