javascript unexpected identifier

I recommend using http://jsbeautifier.org/ - if you paste your code snippet into it and press beautify, the error is immediately visible.

Yes, you have a } too many. Anyway, compressing yourself tends to result in errors.

function () {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("content").innerHTML = xmlhttp.responseText;
    }
} // <-- end function?
xmlhttp.open("GET", "data/" + id + ".html", true);
xmlhttp.send();
}

Use Closure Compiler instead.

In such cases, you are better off re-adding the whitespace which makes the syntax error immediate apparent:

function(){
  if(xmlhttp.readyState==4&&xmlhttp.status==200){
    document.getElementById("content").innerHTML=xmlhttp.responseText;
  }
}
xmlhttp.open("GET","data/"+id+".html",true);xmlhttp.send();
}

There's a } too many. Also, after the closing } of the function, you should add a ; before the xmlhttp.open()

And finally, I don't see what that anonymous function does up there. It's never executed or referenced. Are you sure you pasted the correct code?

Either remove one } from end of responseText;}} or from the end of the line