I've been recently experimenting with prototyping in javascript and I can't figure out why the following code doesn't work. What I would like to do is create a new instance of cheese with parameter n.
function food(n) {
this.n=n;
}
function cheese(n) {
alert(this.n);
}
cheese.prototype=new food;
new cheese('paramesian');
You are creating a new Cheese instance, and the argument n is never used or assigned to the Cheese instance variable this.n, because that logic is only used on the Food constructor.
You can do a couple of things:
1 . Apply the Food constructor inside the Cheese function, using the arguments object and the newly created context (this).
function Food(n) {
this.n=n;
}
function Cheese(n) {
Food.apply (this, arguments);
alert(this.n);
}
new Cheese('paramesian');
2 . Repeat the Food constructor logic (this.n = n) on the Cheese constructor function:
function Food(n) {
this.n=n;
}
function Cheese(n) {
this.n = n;
alert(this.n);
}
Cheese.prototype = new Food();
new Cheese('paramesian');
3 . Use another technique, like power constructors:
function food (n) {
var instance = {};
instance.n = n;
return instance;
}
function cheese (n) {
var instance = food(n);
alert(instance.n);
return instance;
}
cheese('parmesian');
cheese('gouda');
4 . Yet another option, prototypal inheritance:
// helper function
if (typeof Object.create !== 'function') {
Object.create = function (o) {
function F () {}
F.prototype = o;
return new F();
};
}
var food = {
n: "base food",
showName : function () { alert(this.n); }
};
var cheese1 = Object.create(food);
cheese1.n = 'parmesian';
cheese1.showName(); // method exists only in 'food'
Seems like you just want to understand how prototype chaining works in JavaScript. The following is an excellent, simple and well explained tutorial http://www.herongyang.com/JavaScript/Inheritance-from-Constructor-Prototype-Object.html
Edit, This is apparently not prototypical inheritance (see comments), but it does seem to work for this particular purpose.
function food(n) {
this.n=n;
}
function cheese(n) {
this.prototype = food;
this.prototype(n);
alert(this.n);
}
new cheese('paramesian');
this.anything = food; this.anything(n) and you would have still got the correct alert. But cheese by no means entered the prototype chain with that code.
prototype object meant to be used on constructor functions not on object instances (like this is), your example works because when you call this.prototype(n);, you are executing food in the context of the new "cheese" object (this) just like the first example I posted, try to change the prototype keyword on your example for foobar and you'll see that it works also, it has nothing to do with the prototype chain...