Question

I am having problems with the following code:
I have n numbers of select box, whose id are differentiated by the values that the variables ModName and level take:

container.innerHTML = '<form id="listToolForm_'+ModName+level+'"><select id="listToolSelect_'+ModName+level+'" onChange="selValue(this.selectedIndex,'+ModName'+, level)"><option value="0">---</option></select></form>';

The select box is then populated by values taken from a database (this part is not really relevant here). With each form, there is a hidden input with id= idArg'+ModName+'_lev'+level, that will store the index selected by the user, using onChange and the function selValue(x):
Here is the function selValue(index, ModName, level):

function selValue(index, ModName, level){
    document.getElementById("idArg"+ModName+"_lev"+level).value = index;
}

I tried the script setting var ModName = "SL", and var level = 0. I get an error: ‘SL is not defined’. Another test that worked is by replacing the function that fires up onChange:

onChange="selValue(this.selectedIndex)"

and

function selValue(index){
    var level = 0;
    var ModName = "SL";
    document.getElementById("idArg"+ModName+"_lev"+level).value = index;
}

So if the function fired up onChange has one argument, the code works, but does not when using several arguments. I suspect it’s because of the line:

onChange = "selValue(this.selectedIndex,'+ModName'+, level)"

I tried:

onChange="'+window["selValue"](this.selectedIndex, ModName, level)+'"

without success.

Answer 1

The following (I assumed '+ModName'+ was a typo in the question and not in the actual code):
onChange="selValue(this.selectedIndex, ' + ModName + ', level)"
causes JavaScript to substitute ModName with its value, which results in the actual HTML being:
onChange="selValue(this.selectedIndex, SL, level)"
When handling the onChange event, the JavaScript engine “thinks” SL is a variable (hence the error), but you actually want it to be a string (if I understood well).

To turn SL into a string, you should surround it with single quotes. To do that, you will need the \' escape sequence, since you’re already using nested quotes. Simply replace:
onChange="selValue(this.selectedIndex, ' + ModName + ', level)"
with:
onChange="selValue(this.selectedIndex, \'' + ModName + '\', level)"

Also, didn’t you mean:
onChange="selValue(this.selectedIndex, \'' + ModName + '\', ' + level + ')"
?