RefenceError: UET is not defined

Question

I'm struggling to understand why this javascript error keeps popping up on my website and I can't reproduce it. I've followed the bing documentation to install their tracking pixel so I have this in my header :

  <script>
    (function(w,d,t,r,u){var f,n,i;w[u]=w[u]||[],f=function(){
      var o={ti:"xxxxxxxx"};o.q=w[u],w[u]=new UET(o),
              w[u].push("pageLoad")},n=d.createElement(t),n.src=r, n.async=1,
            n.onload=n.onreadystatechange=function(){var s=this.readyState;
              s&&s!=="loaded"&&s!=="complete"||(f(),n.onload=n.onreadystatechange=null)},
            i=d.getElementsByTagName(t)[0],i.parentNode.insertBefore(n,i)})
    (window,document,"script","//bat.bing.com/bat.js","uetq");
  </script>

However in some browsers, the UET variable in this code, which is loaded by the url given as the r variable, seems to not be defined while it's only called when the url is loaded...

If anybody can make sense of it before I reach out to bing, I'll be very thankful!

Answer 1

Building on the accepted answer, I believe there might a way to prevent these errors.

Do you see the bit that says w[u]=new UET(o) in the script? We know that w means window there. And UET is an object created on the global scope (that is, on the global window object), thus it's existence can be tested with window.UET (try running window.UET on the console with and without loading the script, you'll see the former case returns undefined and latter case returns a function function UET(o) {...}. So, technically, you can alter the script putting a conditional there, like the following:

if(w.UET){w[u]=new w.UET(o)}

Here's my attempt to deconstruct the relatively cryptic looking <script> to a more readable function call for better visibility of what's happening there (this may also come handy if you're using it in an SPA like I do):

function initMicrosoftUET(
  w,
  d,
  t = "script",
  r = "//bat.bing.com/bat.js",
  u = "uetq"
) {
  var f, n, i;
  w[u] = w[u] || [];

  f = function () {
    var o = { ti: "xxxxxxxx" };
    o.q = w[u];
    if (w.UET) w[u] = new w.UET(o) || [];
    w[u].push("pageLoad");
  }

  n = d.createElement(t);
  n.src = r;
  n.async = 1;
  n.onload = n.onreadystatechange = function () {
    var s = this.readyState;

    (s && s !== "loaded" && s !== "complete") || f();

    n.onload = n.onreadystatechange = null;
  }

  i = d.getElementsByTagName(t)[0];
  i.parentNode.insertBefore(n, i);
};

initMicrosoftUET(window, document);

Answer 2

Ad/tracker blocking.

If the user blocks the domain bat.bing.com, then UET never gets defined.

For example, FireFox console on accessing a website that contributes to Bing tracking while blocking such things:

---

As for 'fixing' it, in my view the most appropriate fix is to ignore it*. If you're seeing it in Sentry.io or similar, the single occurrence can be ignored rather than all undefined variable errors (which of course you wouldn't want to blanket-ignore).

The user is deliberately blocking it, and the error isn't user-visible, (without opening the console) so nothing to worry about.

_{*Of course, you could always not load trackers for any of your users ;)}