Question

Let’s say I have the following code snippet.

function test(id) { alert(id); }

testChild.prototype = new test();

function testChild(){}

var instance = new testChild('hi');

Is it possible to get alert('hi')? I get undefined now.

Answer 1

That’s how you do this in CoffeeScript:

class Test
  constructor: (id) -> alert(id)

class TestChild extends Test

instance = new TestChild('hi')

Nope, I’m not starting a holy war. Instead, I’m suggesting to take a look at resulting JavaScript code to see how subclassing could be implemented:

// Function that does subclassing
var __extends = function(child, parent) {
  for (var key in parent) {
    if (Object.prototype.hasOwnProperty.call(parent, key)) {
      child[key] = parent[key];
    }
  }
  function ctor() { this.constructor = child; }
  ctor.prototype = parent.prototype;
  child.prototype = new ctor;
  child.__super__ = parent.prototype;
  return child;
};

// Our code
var Test, TestChild, instance;

Test = function(id) { alert(id); };

TestChild = function() {
  TestChild.__super__.constructor.apply(this, arguments);
}; __extends(TestChild, Test);

instance = new TestChild('hi');

// And we get an alert

See it in action at http://jsfiddle.net/NGLMW/3/.

To stay correct, the code is slightly modified and commented to be more readable, compared to CoffeeScript output.