How to convert a javascript string to a javascript function, preserving the correct "this"?

Question

A browser element has an onpopupshowing attribute, which is a string consisting of several javascript statements.

I want to "override" that bit of javascript.

Were this a function, I'd just assign it to a variable, assign my "overriding" function to the element attribute, and (if need be) call the original function from my overriding one, pretty much like this:

var oldfunc;
function myoverrridingfunc () { my statement; oldfunc(); my other statement; }

oldfunc = element.getAttribute( "onpopupshowing" )
element.setAttribute( "onpopupshowing", myoverrridingfunc );

but element.getAttribute( "onpopupshowing" ) isn't a function, it's a string.

I could use the Function object to turn the string into a function, but the string includes references to this. How do I preserve the right this?

What's the best way of intercepting/overriding this bit of script? Note: this only needs to work in Firefox, not in any version of java/ECMAscript.

Thanks.

Answer 1

var oldfuncStr = element.getAttribute("onpopupshowing");
element.removeAttribute("onpopupshowing");
var oldfunc = new Function(oldfuncStr);
element.addEventListener("popupshowing", function()
{
  myStatement;
  oldfunc.call(this);
  otherStatement;
}, false);

call is specifically for specifying this. I tested in Firefox, but with onclick event rather than onpopupshowing.

Answer 2

Perhaps you are overthinking this. Why not just wrap your JavaScript statements in quotes and assign it to the attribute? If the attribute expects JavaScript statements as a string, I wouldn't expect that assigning a function reference would even work anyway. (does it?)