function Foo() {}
Foo.prototype.bar = "baz";

var foo = new Foo();
console.log(foo.bar); //-> "baz"
console.log(Foo.prototype.bar); //-> "baz"

foo.bar = "qux";
console.log(foo.bar); //-> "qux"
console.log(Foo.prototype.bar); //-> "baz"

var qux = new Foo();
console.log(foo.bar); //-> "qux"
console.log(qux.bar); //-> "baz"
console.log(Foo.prototype.bar); //-> "baz"

Foo.prototype.bar = "quo";
console.log(foo.bar); //-> "qux"
console.log(qux.bar); //-> "quo"
console.log(Foo.prototype.bar); //-> "quo"