JavaScript - overriding toString prototype

There was a question on here already about overriding toString and the answer is this:

function Foo() {}

Foo.prototype.toString = function()
{
    return "this is Foo";
}

however my question is:

is there a way to put this prototype override INSIDE of the object declaration like so:

function Foo() {
    Foo.prototype.toString = function(){
        return "this is Foo";
      }
   }

or like so:

var Foo =  {
Foo.prototype.toString = function(){
    return "this is Foo";
   }
}

it's really weird/annoying to have to put the prototype outside of the object declaration, but maybe this is a quirk of JavaScript. I have been wondering this for awhile.