原型对象 Javascript

Question

When I make a prototype object, and use it for a new instance I should expect the name of the object is equal to how I called it. My prototype is called AUTO and the new one is called BMW

var auto = function(a,b,c) {

    this.a = a;
    this.b = b;
    this.c = c;

};

var bmw = new auto("blue", true, 123);

console.log(bmw);

So the outpunt of this is: auto {a: "blue", b: true, c: 123}

But I expected BMW {a: "blue", b: true, c: 123}

However when I trigger this piece of code console.log(bmw.a); i receive the correct value.

So my question is: Why is BMW = auto {a: "blue", b: true, c: 123} and not tot BMW {a: "blue", b: true, c: 123}

Answer 1

Your BMW is an instance of your prototype auto . Its name, BMW , is just the variable in which this instance is stored. Try this for example:

 var auto = function(a,b,c) { this.a = a; this.b = b; this.c = c; }; var bmw = new auto("blue", true, 123); document.write('<br />What is BMWs type? '); document.write(typeof bmw); document.write('<br />What is autos type? '); document.write(typeof auto); document.write('Is BMW an instance of auto? '); document.write(bmw instanceof auto ? 'Yes' : 'No');

In short, bmw is the variable that is holding an instance of your prototype Auto - it is not a new prototype. Auto is the constructor function - it's type is Function . Prototypes (or classes in different languages) are simply general objects that define the structure and methods of a certain object by default. For instance, you could make a prototype for 'vehicle', and then derive from those two prototypes for two-weeled vehicles and four-wheeled. Then you could make a new four wheeled vehicle, but this time you are making an actual vehicle, giving it a name, etc...

If you want to make a specific prototype for BMW, you should inherit it using the right methods:

var bmw = function(a,b,c) {
    // Using `call` will run the constructor of auto, with the context of `this`
    auto.call(this, a, b, c);
    // Then we can do prototype specific things here
    this.brand = 'BMW';
};
var bmw = new bmw("blue", true, 123);

Once you add prototype methods, btw, you will also have to use Object.create(auto.prototype) to chain it correctly.

Check out MDN for more information - have a good read there: https://developer.mozilla.org/en/docs/Web/JavaScript/Inheritance_and_the_prototype_chain