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Frequently Asked Question #14

Question: What is this HM_f_DoNothing code, and why must I insert it at the top of each of my configuration files?

Answer: As explained as part of step 2 of the setup instructions, every configuration file must begin with the following block of code:

function HM_f_DoNothing() {return;}
if(typeof(HM_f_UpdateDefaults)=="undefined") {
	HM_f_UpdateDefaults=HM_f_DoNothing;
	HM_f_SetMenuTemplate=HM_f_DoNothing;
	HM_f_SetMenus=HM_f_DoNothing;
	HM_f_SetItems=HM_f_DoNothing;
}

And while this step is covered in the setup instructions, the full reason why this code is necessary is not. That we'll provide now.

When loading the HierMenus scripts into the page, we use a two step process. First, HM_Loader.js is loaded into the page, by way of the standard HTML tag that you must insert into each of your HierMenus pages:

<script type="text/javascript"
        language="JavaScript1.2"
        src="/hm/scripts/HM_Loader.js">
</script>

Within HM_Loader.js, two (or more) files are additionally loaded into the page using standard document.write JavaScript statements. When loading scripts into a page via this method, however, each of the two latest major browsers (Internet Explorer and Mozilla/Gecko) can process the scripts slightly differently than they would if the scripts were loaded directly within the page (as HM_Loader.js is) or if the scripts were simply embedded within the page. The key difference is this: in both browsers, when loading multiple scripts into a page via document.write statements (as we do in HierMenus), if the loading process of the page is stopped (via the user clicking the stop button or the user clicking through to a new page) there is the possibility that the browser will actually parse and execute one of the later external scripts--even though a prior script never finished loading--just before the page is actually unloaded. In the case of HierMenus, our second script--namely, your configuration file--relies on routines that are defined within the first script loaded--namely the HierMenus execution scripts. If the second JavaScript is parsed without the benefit of having the first JavaScript already parsed and executed, the results will be errors as the configuration file commands attempt to execute functions that, as far as the browser is concerned, don't actually exist.

Presumably, this later-before-former JavaScript parsing only happens when the two scripts in question are of significantly different sizes. When scripts are loaded into a page via document.write commands, most browsers can make the overall download of the page more efficient by retrieving both scripts simultaneously, in the same manner that multiple images of a page can be retrieved simultaneously. Though multiple retrieval requests are made for the scripts on the page, the actual processing of the scripts (and parsing of any further HTML) is delayed, so that the scripts can be executed in the proper order. In this one case, however, if the retrieval of the first script is interrupted before it is completed but after the retrieval of the second script is complete then the second script can be quickly parsed and executed after the stop button is clicked or just before the current page is unloaded and the new page is loaded.

There are many ways to work around this problem, and we've chosen a way that we thought would be the easiest to explain and implement for all HierMenus configurations. Unfortunately, it's impossible for us to fix the problem using code within the HM_Loader.js or the execution scripts; since the fact is, at the point the problem occurs it's possible that neither of them will be available to the configuration file commands. (Gecko, for example, will occasionally throw an error at a point after the HM_Loader.js code has already been removed from the page.) Basically, the HM_f_DoNothing code above provides "dummy" function declarations for each of the main commands in the configuration file if and only if they are not already declared. In this way, if the execution scripts never finished loading, the commands in the configuration file will still be executed, but they will simply "do nothing." No menus will be created (as was the user's intention, since they clicked the stop button), but no errors will be generated, either.

Of course, if you would like to implement your own checks based on the information above, you are welcome to do so.