A CodePen by Chris Roberts.

script.js

var myobjs = [
	{position: null},
	{position: null},
	{position: 5}, 
	{position: 1}, 
	{position: null},
	{position: 6}, 
	{position: 3}, 
	{position: 10}, 
	{position: 2}, 
	{position: 11}, 
	{position: null},
	{position: 4}, 
	{position: null},
	{position: 15},
	{position: null},
	{position: 0}
];

var nextAvailable,
  j, i, it, found;

for (i = 0; i < myobjs.length; i++) {
  for (j = 0; j < myobjs.length; j++) {
    it = myobjs[j];
    if (it.position && it.position === i) {
      found = i;
    }
  }
  if (found === i - 1) { 
    nextAvailable = i;
    break;
  }
}

console.log(nextAvailable);