js中构造函数与原型

f1._proto_==Foo.prototype//true

function Foo() { } ;
var f1 = new Foo();

Foo.prototype.x = "hello";
f1.x   //=> hello
Foo.x //=> undefined

function foo() { }
f1 = new foo();
f2 = new foo();
foo.prototype.x = "hello";

f1.x  => "hello"
f2.x  => "hello";

f1.x = "goodbye";   //setting f1.x hides foo.prototype.x

f1.x  => "goodbye"  //hides "hello" for f1 only
f2.x  => "hello"

delete f1.x
f1.x  => "hello";   //foo.prototype.x is visible again to f1.