Question

Im trying to set up a variable and pass it constructor function essentailly like an anonymous type in c# but javascript doesnt seem to like it…

var aPerson = function Person(){

};


$(document).ready(function(){

  Person.prototype.age = 22;

  window.alert(aPerson .age);

});

Why cant I do this?

Answer 1

Person is only declared as part of your aPerson variable, but it needs to be defined explicitly (e.g. function Person(){}) before it can be used for prototypical inheritance. You need something more like this:

// Create a Person function object
function Person() {

};

// Modify that function's prototype
Person.prototype.age = 22;

$(document).ready(function() {
  // aPerson.__proto__ = Person.prototype
  var aPerson = new Person();

  // JS checks whether aPerson has age already set, if not, it checks
  // aPerson's prototype -- in which case it's given the value 22
  alert(aPerson.age);
});

Here’s the deal: the property prototype works together with new by copying the prototype reference to the object (you can see what that entails by running console.dir(aPerson) in Chrome console, for example). JavaScript checks the original object itself first, then prototype to see whether a function or property exists. That means you can change the reference prototype age later and see the changes reflected in the original object. Also, you can declare your own age in the original object itself and have that override the prototype.