IE9 this = DispHTMLWindow2 inside a prototype's function

I created an extension function called startsWith in my general js.

It goes like this:

String.prototype.startsW开发者_StackOverflowith = function (str) {
 if (this.indexOf(str) == 0) {
     return true;
 }
 return false;
}

When running in IE9 i get an error saying: "Object doesn't support property or method 'indexOf'".

When looking in the debugger it seems like this is DispHTMLWindow2.

Any help?

Thanks.

---

You could try the following as taken from this site

String.prototype.startsWith = function(str) 
{return (this.match("^"+str)==str)}

String.prototype.endsWith = function(str) 
{return (this.match(str+"$")==str)}

String.prototype.trim = function(){return 
(this.replace(/^[\s\xA0]+/, "").replace(/[\s\xA0]+$/, ""))}