JavaScript：ECMAScript5中的继承

Question

var A = function () {
    this.p1 = 2;
};
A.prototype.f1 = function () {
    return 7;
};
var B = function () {
    inherit(A, B);
};


function inherit(Child, Parent) {
    Child.prototype = Object.create(Parent.prototype);
    Child.prototype.constructor = Child;
}

var b = new B();
console.log(b.p1); // get undefined here 

I am new to JS, sorry for dump question. I would like to inherit B from A . What am I doing wrong?

Answer 1

You're only calling inherit() after creating the instance of B .

You need to call inherit() statically, once, after defining both functions.

You also need to call A on your instance in B .

For more details on how to properly do inheritance, see my blog .

Answer 2

What am I doing wrong?

Two things:

1. You're calling inherit inside B . You should be doing it outside.

2. Inside B , you should be calling A , eg

    A.call(this/*, other, args, here, if, needed*/); 

   or

    A.apply(this, arguments); 

   to pass on all of the arguments B received at runtime via the automatic arguments pseudo-array.

Like so:

 var A = function () { this.p1 = 2; }; A.prototype.f1 = function () { return 7; }; var B = function () { A.call(this); // <==== Added }; inherit(A, B); // <==== Moved function inherit(Child, Parent) { Child.prototype = Object.create(Parent.prototype); Child.prototype.constructor = Child; } var b = new B(); console.log(b.p1); // get 2 here now 

Answer 3

You didn't call the base constructor. Also, it's enough if you inherit only once the classes.

 var A = function () { this.p1 = 2; }; A.prototype.f1 = function () { return 7; }; var B = function () { A.apply(this, arguments); }; inherit(A, B); function inherit(Child, Parent) { Child.prototype = Object.create(Parent.prototype); Child.prototype.constructor = Child; } var b = new B(); console.log(b.p1); // get undefined here