Question

The following code doesn’t produce a prototype like I thought it might. Can anyone see what I am doing wrong?

var A = function () {
    return {
        workingTest: function () {return "OK";}
    };
};

A.prototype.notWorkingTest = function () {return "Not so much";};

var a = new A();

a.workingTest(); // "OK"
a.notWorkingTest(); // TypeError: "undefined_method"

Any ideas? I thought that this was the right way to extend JS classes, but I am missing something.

Answer 1

Change A to

var A = function () {
  this.workingTest = function () {return "OK";};
};

The problem is that your original A function was creating and returning a direct instance of Object instead of using the instance of A that was created by the new operator and bound to this.

To understand this, try running

var B = function () { return { x: 42 }; }
var C = function () { this.x = 42; }
var b = new B;
var c = new C;
alert("b.constructor === B : " + (b.constructor === B));  // false
alert("b.constructor === Object : " + (b.constructor === Object));  // true
alert("c.constructor === C : " + (c.constructor === C));  // true

So because B returns a new object, the value it returns is no longer an instanceof B and does not use B‘s prototype.