Question

Hi all I have been learning coffeescript from the book Mark Bates programming in coffeescript pdf I have been banging my head off at the behavior of javascript even though both seems to have same implementation

Example-1

class Employee
  constructor: (@attributes)->
    for key, value of @attributes
      @[key] = value
  printInfo: ->
    alert "Name: #{@name}"
emp1 = new Employee
  name: "Mark"
  printInfo: ->
    alert "Hacked ur code !"

emp1.printInfo()

Corresponding javascript

var Emp, Employee, emp1, emp2;

Employee = (function() {

  function Employee(attributes) {
    var key, value, _ref;
    this.attributes = attributes;
    _ref = this.attributes;
    for (key in _ref) {
      value = _ref[key];
      this[key] = value;
    }
  }

  Employee.prototype.printInfo = function() {
    return alert("Name: " + this.name);
  };

  return Employee;

})();

emp1 = new Employee({
  name: "Mark",
  printInfo: function() {
    return alert("Hacked ur code !");
  }
});

emp1.printInfo();

 This alerts "Hacked ur code !"

Example-2

class Emp
  constructor: (@attributes)->
  printInfo: ->
    alert "Name: #{@attributes.name}"
emp2 = new Emp
  name: "Mark"
  printInfo: ->
    alert "Hacked ur code"
emp2.printInfo()

Corresponding javascript

Emp = (function() {

  function Emp(attributes) {
    this.attributes = attributes;
  }

  Emp.prototype.printInfo = function() {
    return alert("Name: " + this.attributes.name);
  };

  return Emp;

})();

emp2 = new Emp({
  name: "Mark",
  printInfo: function() {
    return alert("Hacked ur code");
  }
});

emp2.printInfo();

 This alerts "Name: Mark"

Where is the difference ?

Answer 1

In the first example, all properties of the object you pass to the constructor (attributes) are added to the current instance (that’s what the loop does). Instance properties hide prototype properties, that’s why the printInfo function you passed to the constructor is executed. You can access the original method by using Employee.prototype.printInfo.call(emp1);

In the second example, nothing like this happens. The attributes object just lives inside the attributes property of the instance. To get a different alert, you need to write emp2.attributes.printInfo();