Creating stable sort for JavaScript issues in IE11

I am trying to create a stable sort element to the default JavaScript .sort() function. I have it working in all browsers except for IE11 and below.

Here is the code:

   Array.prototype.stableSort = function(cmp) {

    cmp = !!cmp ? cmp : (function(a, b) {
      if (a < b) return -1;
      if (a > b) return 1;
      return 0;
    });
    let stabilizedThis = this.map(function(el, index) { return [el, index]; });

    stabilizedThis.sort(function(a, b) {
      let order = cmp(a[0], b[0]);
      if (order != 0) return order;
      return a[1] - b[1];
    });

    for (let i=0; i<this.length; i++) {
      this[i] = stabilizedThis[i][0];
    }
    return this;
  }

For reference, the above code fails even when I am not actually using this stable sort functionality in my code. My usage will be something like this:

  sortedArray.stableSort(function(a,b) {
    if (type == "string") {
      return a[index]["value"].toString().localeCompare(b[index]["value"].toString());
    } 
    else if (type == "number") {
      return a[index]["value"] - b[index]["value"];
    }
  });

Note: to narrow done the issue, I have discovered that -- at a minimum, this code works in IE11:

Array.prototype.stableSort = function(cmp) {

  cmp = !!cmp ? cmp : (function(a, b) {
    if (a < b) return -1;
    if (a > b) return 1;
   return 0;
  });

  // everything here was removed for testing

  return this;
}

Of course this does not sort (or stable sort), but it does not cause a syntax error.

Unfortunately, the development tools and console does not give me an indication of the line number where the failure is.

For reference, I am basing my code off of what I found at this link. They were using stuff that wasn't compatible with ES5 (IE11's limit) so I had to change it up a bit. Not sure if I missed something else with this.

Any ideas on what is happening?

Microsoft IE11 does not support ES6, like let statements.

You could replace it with var statements.

Array.prototype.stableSort = function (cmp) {
    cmp = cmp ||function (a, b) {
        if (a < b) return -1;
        if (a > b) return 1;
        return 0;
    };

    var stabilizedThis = this.map(function (el, index) { return [el, index]; });

    stabilizedThis.sort(function (a, b) {
        return cmp(a[0], b[0]) || a[1] - b[1];
    });

    for (var i = 0; i < this.length; i++) {
        this[i] = stabilizedThis[i][0];
    }
    return this;
}