Prototypal inheritance in javascript, how does it work?

Question

This is my code:

var Quo = function(string) {            //This creates an object with a 'status' property.
    this.status = string;
};
Quo.get_status = function() {
    return this.status;
}
Quo.get_status = function() {
    return this.status;
}

var myQuo = new Quo("confused");        //the `new` statement creates an instance of Quo().

document.write(myQuo.get_status());     //Why doesnt the get_status() method attach to the new instance of Quo?

When I run this code the result is [object Object]. My question what properties of a constructor are inherited by an instance?

Matt · Accepted Answer · 2011-08-04 17:12:49Z

2

My question what properties of a constructor are inherited by an instance?

Anything in Quo.prototype will be available to the instance.

This code should make your example work:

Quo.prototype.get_status = function() {
    return this.status;
};

Quo.prototype.get_status = function() {
    return this.status;
};

7 Comments

dopatraman Over a year ago

@Matt--what properties are contained in the prototype object by default?

Matt Over a year ago

@codeninja - it's a chain. By default you have everything from Object.prototype, but it is possible to "extend" from others. Take a peek at the MDC articles I linked

dopatraman Over a year ago

@Matt--so when Quo is created, Quo.prototype is empty? Or does it contain the properties of Quo? note: i havent looked at the MDN articles yet.

Matt Over a year ago

@codeninja - when you create Quo itself (NOT the instances), the Quo.prototype is technically empty. Prototypes are chains, so when a quo instance doesn't find what it wants, it looks in the next prototype in the chain (in this case, it looks in Object.prototype).

Matt Over a year ago

@codeninja - np. on your way out, just to give you an idea of the flexibility of prototypal inheritance .. prototypes are checked dynamically at property-lookup-time. So you can modify Quo.prototype at any time and all instances (future and existing) will be affected.

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