Firefox 3 javascript evaluating order problems

I found out the hard way the recently released third main version of Firefox has a serious issue related to the order in which the Javascript code is executed in a page. The bug is somehow related to the files being, or not, cached on the browser.

Wonder Ajax framework adds a lot of Ajax goodies to a standard WebObjects applications. The way a page is built is by placing on the page header all the calls to load Javascript files. There’s a special case, which is when you need a JSON proxy. That is useful if you need to write more powerful Ajax behaviour than all the component trickery Wonder offers you. The JSON proxy will not only load the necessary Javascript file, but run one code line to create the actual proxy and establish the communication with the server. So the page header will have something that looks more or less like this:

<script src="/some/path/prototype.js"></script>
<script src="/some/path/effects.js"></script>
<script src="/some/path/wonder.js"></script>
<script src="/some/path/english.js"></script>
<script src="/some/path/combo.js"></script>
<script src="/some/path/feedback.js"></script>
<script src="/some/path/jsonrpc.js"></script>
<script type="text/javascript">
var jason = new JSONRpcClient("proxyUrl");
</script>
<script src="/some/path/comboboxes.js"></script>
...

When the page loads, all those scripts will be loaded and the new JSONRpcClient() method will be executed during the page load, as expected.

The problem starts when you have a javascript method associated with the onLoad window event. This is done by using the onload attribute in the body element:

<body onLoad="initPage();">

The onLoad function should be triggered when the page finishes loading. This, implicitly, means that the onLoad will be run only after all the inline javascript code (in the page header or body) be run, because that is still considered to be part of the page loading process. So, in our case, first, load all the JS and create the JSON proxy, and then run the onLoad method.

This works on every browser, and also on Firefox 3 on the first page load. The problem is that most of the subsequent page loads on Firefox 3 won’t work, at least if your onLoad method depends on the existence of the JSON proxy. For some reason, when Firefox 3 has all the Javascript files already in the local cache, the onLoad event is triggered too soon, namely before the header scripts are executed and the proxy has been created. This will break all your code executed by the onLoad method that assumes the proxy is already ready to work.

I have tried several solutions for this. The only one I had success with is the following. It’s kind of dirty, but it works. The idea is simple: if the onLoad method is to be executed before the proxy exists, than we wait some miliseconds and try again later until the proxy is ready to be used. So, if your method is like this:

function myOnloadMethod() {
  runInitStuff();
}

Change it to:

function myOnloadMethod() {
  if( window.jason ) {
    runInitStuff();
  } else {
    setTimeout( 'myOnloadMethod();', 10 );
  }
}

This will check every 10 milliseconds if the proxy is already there. If it is, your method will be run. If not, it will keep trying.

3 thoughts on “Firefox 3 javascript evaluating order problems

  1. matt w

    Wow – this is the only Google result I’ve found on this problem. Seems like kind of a huge issue. The code in SCRIPT tags gets executed out of order on subsequent loads. How is this not breaking tons of sites? Have you found any verification of this or response to it from the FF team?

    Reply
  2. Pingback: FireFox 3: the evaluation order bug is back « Terminal.app

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