将函数原型设置为null对新创建的javascript对象无效

Question

I am using the following code:

function Tmp(){}
var tmp = new Tmp()
Tmp.prototype.f1 = function(){console.log("f1");}

tmp.f1() -> output: f1 
Tmp.prototype.f1 = function(){console.log("f1 update");}
tmp.f1() -> output: f1 update

//this is confusing
Tmp.prototype = null;
tmp.f1() -> output still: f1 update

My question is: when I set Tmp.prototype = null; why it has no effect to tmp.f1(). In other words, the tmp.f1() still output the same result. My understanding is that tmp.f1() will recursively check the property within the prototype chain and if the method is available, then the method is called. But by setting Tmp.prototype = null, I expect that tmp.f1 will be undefined, but it's not the case.

Thanks.

Answer 1

Probably the best reference in the specification is to 9.1.14 OrdinaryCreateFromConstructor .

However, you can test this yourself.

 // Constructor, has a default prototype object that // is a plain Object function Foo(){} // Instance has its internal [[Prototype]] set to the // constructor's prototype when it's created var foo = new Foo(); // See? document.write(Object.getPrototypeOf(foo) === Foo.prototype); // true // Keep reference to Foo.prototpye var originalFooProto = Foo.prototype; // Change Foo.prototype Foo.prototype = {}; // Create another instance foo2 = new Foo(); // Current Foo.prototype is not original document.write('<br>' + (originalFooProto === Foo.prototype)) // false // foo's internal prototype is not Foo's new prototype document.write('<br>' + (Object.getPrototypeOf(foo) === Foo.prototype)) // false // foo's internal prototype is still the old Foo.prototype document.write('<br>' + (Object.getPrototypeOf(foo) === originalFooProto)) // true // foo2's internal prototype is the new Foo.prototype document.write('<br>' + (Object.getPrototypeOf(foo2) === Foo.prototype)) // true