Question

I have this type of code for prototypal inheritance in Javascript.

function A() {
    this.a = 1;
}
function B() {
    someEl.innerHTML(this.a);
}
B.prototype = new A();

However, I am wondering if there is a better way to do it, preferably one that brings the prototype declaration into the declaration of B.

I tried setting this.prototype in B, but it just showed this.a as undefined.

So, is there a better way to do this?

Answer 1

The prototype property should be used only in constructor functions, this.prototype doesn’t makes sense because this just the new object instance, not the constructor.

Assign the prototype inside the constructor itself isn’t a good idea, since that assignment will be executed each time you create a new object (by new B();).

Your code seems perfectly valid to me, the only thing that you should be aware of, is that if you replace the prototype property of a constructor, like you do it with B.prototype = new A(); the constructor property of the object instances of B, will point to A.

Usually is recommended to restore that property, e.g.:

function A() {
    this.a = 1;
}

function B() {
    someEl.innerHTML(this.a);
}

B.prototype = new A();
B.prototype.constructor = B; // restore the `constructor` property

var foo = new B;
foo.constructor === B; // true

Give a look to:

• Constructors considered mildly confusing (article)
• Setting javascript prototype function within object class declaration (SO question)