Javascript-构造函数的属性和原型

Question

I'm trying to understand one thing about constructors. Namely I don't understand how function object's properties, aka constructor's properties can't be accessed using Foo.prototype.property, example:

//let's create new function object with predefined properties
function Foo() {
    this.name = "Mike"; 
    this.returnName = function() {
                       return 'This persons name is ' + this.name + '.';
                      };
};

//now let's create new object using constructor and prototypal inheritance
var mike = new Foo();

mike.returnName(); //returns "This persons name is Mike."

I understand that mike inherits properties from Foo's prototype. So these properties must be present inside of the prototype. new object points toward the prototype with internal prototype link using __ proto __. I get that. Thing that I can't get around is how this doesn't work:

Foo.prototype.returnName();

Constructor has .prototype link with prototype property and vice versa prototype has .constructor link with the Foo.

I know that calling the Foo(); function first then calling for window.returnName(); would return the sentence, since this context is set to window.

Is this because when I call the method using prototype it assigns it to the window? Is there any way I could access/call this constructor's property?

Answer 1

You are not specifying returnName on the prototype of Foo. If you would do it like this:

function Foo(){
    this.name = "Mike"; 
}

Foo.prototype.returnName = function(){
   return 'This persons name is ' + this.name + '.';
};

var mike = new Foo();

mike.returnName();

Things would be different.

Answer 2

The function returnName is not assigned to the prototype of Foo . Instead it's assigned to every instance of Foo , just like the name is "Mike" .

Whenever you call new Foo() a new context is created, this is the this context of that instance.

If you call Foo() directly (without new ) this is the global object ( window ).