Idiot-proofing JavaScript "classes"

Given this JavaScript "class":

function Person(first,last)
{
    this.name=first+' '+last;
}

//Instantiation
var superstar=new Person('Wayne','Rooney');

A correct instantiation (like above) will set the context for this to be superstar.

But what if someone (or even you) wrongly instantiate it as below?

//"new" is omitted
var genius=Person('Wolfgang Amadeus','Mozart');

Well, in that case, this will refer to window and you've just set or overrode window.name. That's bad.

To prevent this, you should check if the instance of this is that of the function, like below:

function Person(first,last)
{
    if(!(this instanceof Person))
    {
         return new Person(first,last);
    }
    this.name=first+' '+last;
}

This doesn't make omitting new right, but it does make it harmless as this will always refer to the context it's supposed to.

Note:

Another way of doing the if-check, but in a more dynamic way, would be to use this instanceof arguments.callee.

However, ES5 forbids use of arguments.callee in strict mode.