JavaScript inheritance: when constructor has arguments

Question

Using pure JavaScript to do inheritance, this is what I usually do:

function A() {}
A.prototype.run = function () {};

function B() {}
B.prototype = new A;
B.prototype.constructor = B;

Since there is no arguments to pass into the constructor, new A has nothing to complain about. Now, I haven't figured out a good way to do inheritance if the constructor has arguments to pass. For example,

function A(x, y) {}
A.prototype.run = function () {};

function B(x, y) {}
B.prototype = new A;
B.prototype.constructor = B;

I could pass some arbitrary values like:

B.prototype = new A(null, null);

In some cases, I may need to validate x and y in the constructor of A. In some extreme cases, I need throw errors when checking x or y. Then, there is no way for B to inherit from A using new A.

Any suggestions?

Thanks!

Answer 1

Well, if you want to make B.prototype an object that inherits from A.prototype, without executing the A constructor, to avoid all possible side-effects, you could use a dummy constructor to do it, for example:

function tmp() {}
tmp.prototype = A.prototype;
B.prototype = new tmp();
B.prototype.constructor = B;

You could create a function to encapsulate the logic of the creation of this new object, e.g.:

function inherit(o) {
  function F() {}; // Dummy constructor
  F.prototype = o; 
  return new F(); 
}

//...
B.prototype = inherit(A.prototype);
B.prototype.constructor = B;

If you target modern browsers, you could use the ECMAScript 5 Object.create method for the same purpose, e.g.:

B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
//..