This mergesort should "of" failed, right?

I noticed something weird while reviewing this mergesort implementation on Code Review…

/************************************************************
 * Mergesort implementation
 ***********************************************************/

function sort(array) {
  var len = array.length;
  var middle = Math.floor(len*0.5);
  var left = array.slice(0,middle);
  var right = array.slice(middle, len);

  if (len == 1) {
    return array;
  } else {

  }

  return merge(sort(left), sort(right));
}


function merge(left, right) {
  var a = left.length;
  var b = right.length;


  if (a > 0 && b > 0) {
    if (left[0] > right[0]) {
      return [].concat(left[0], merge(left.slice(1,a), right));
    } else {
      return [].concat(right[0], merge(right.slice(1,b), left));
    }
  } else if (a == 0) {
    return right;
  } else of (b == 0)
    return left;

}


/************************************************************
 * Demonstration
 ***********************************************************/

function doSort() {
    var array = document.getElementById('in').value.split(/[, ]+/).map(function(e) {
        return parseInt(e);
    });
    var sorted = sort(array);
    document.getElementById('out').value = sorted;
}

function generateRandom(len) {
    var array = [];
    for (var i = 0; i < len; i++) {
        array.push(Math.round(Math.random() * 100));
    }
    document.getElementById('in').value = array;
};

generateRandom(20);

<button onclick="generateRandom(20)">⬇︎ Generate random numbers ⬇︎</button>
<div><input id="in" size="80"></div>
<button onclick="doSort()">⬇︎ Sort ⬇︎</button>
<div><input id="out" size="80" disabled></div>

The last conditional branch is else of rather than else if. Normally, else of should result in a syntax error. Yet, no matter how hard I try, I can't trigger the syntax error — it always successfully returns an array sorted in descending order!

I know, else of (b == 0) could just be replaced by else, but still, I want to know: How could this code possibly work?