Scripting with bridge

Question

Hello, i am pretty new to this so i am still figuring out how this scripting works
In general what i want to do is generate a jpg image from an symbols.ai library file and because bridge shows this library in some way, i want to use a script against bridge.
So now i have found a script from the sdk - SnpSaveAsJPEG and i am trying to understand what is happening because i have no experience with javascript.
maybe some advice on some good literature is also good?
can someone please help me with the following questions:
at the bottom we have "new SnpSaveAsJPEG().run();" that makes the program run
why is it executing the (at the top) "function SnpSaveAsJPEG()" even if you change that functions name?
why do we set requiredContext?
is the "$.level" always needed and why do we set it?
next we have the method "SnpSaveAsJPEG.prototype.run = function()"
where does "prototype" come from?
Why is it executing this function as there is nowhere a reference to this?
thanks

Answer 1

Hello Muppet Mark.
We are using CS 5.5
Here is the sample code, as found in the sdk(but some little test modifications by me), copyright adobe
The script is working correct, but i am trying to understand how it does what it does.
#target bridge
#targetengine "MainEngine"
function SnpSaveAsJPEG()
    //The context in which this snippet can run.
     //type String
    this.requiredContext = "\tExecute against Bridge.\nBridge must be running and \n"
        + "at least one thumbnail selected that is not a folder.";
    $.level = 1; // Debugging level
Functional part of this snippet. 
Gets a File object for each selected thumbnail, and uses it to
create a BitmapData object. Uses the BitmapData object to export
to the JPEG format.
     @return True if the snippet ran as expected, false if no files are selected in Bridge
     @type Boolean
SnpSaveAsJPEG.prototype.run = function()
    var retval = true;
    if(!this.canRun()) {
        retval = false;   
        return retval;
    // Get the selected Thumbnail objects - only accept these file types
    var thumbs = app.document.getSelection("ai");
    // Go through each of the selected thumbnails
    if(thumbs.length != 0)
        for(var i = 0;i < thumbs.length;i++)
            // get associated File object, ignore folders
            if(thumbs[i].spec instanceof File)
                var thumb = thumbs[i];
                // create a BitmapData object
                var bm = new BitmapData(thumbs[i].spec);
                if(bm instanceof BitmapData)
                    // create the path and file name
                    var exportFilePath = thumbs[i].parent.spec + "/SnpSaveAsJPEG_" + thumbs[i].name + ".jpg";
                    // create the new file and export the data
                    bm.exportTo(new File(exportFilePath));
    else
        retval = false;
    return retval;
Determines whether snippet can be run given current context.  The snippet will
fail if these preconditions are not met:
<ul>
<li> Must in Bridge
<li> At least one file must be selected in Bridge
</ul>
@return True if this snippet can run, false otherwise
@type boolean
SnpSaveAsJPEG.prototype.canRun = function()
    // must run in Bridge
    // must be at least one selection.
    if((BridgeTalk.appName == "bridge") && (app.document.selectionLength > 0))
        return true;
    // Fail if these preconditions are not met. 
    $.writeln("ERROR:: Cannot run SnpSaveAsJPEG");
    $.writeln(this.requiredContext);
    return false;
"main program": construct an anonymous instance and run it
  as long as we are not unit-testing this snippet.
//if(typeof(SnpSaveAsJPEG_unitTest)  == "undefined") {
//    new SnpSaveAsJPEG().run();
new SnpSaveAsJPEG().run();