Question

Is it possible to check whether a browser-provided function has been replaced by user code?

function alert(str){
    document.write(str);
}

alert('hello world');

Answer 1

It is impossible to know if browser methods are overwirtten or not. e.g.

function foo(){
  return 'foo was called';
}

foo.toString = function() {
                 return 'function alert() {\n    [garbage code]\n}';
               };
var a = window.alert;
window.alert = foo; 

a(window.alert);  
document.write(alert);

About all you can say is that if you don’t get the default response, the method was probably modified since the toString method of host methods is generally read only. But if you get the default, you don’t know if it was modified or not.