How does JavaScript .prototype work?

I’m not that into dynamic programming languages, but I’ve written my fair share of JavaScript code. I never really got my head around this prototype-based programming, does any one know how this works?

I remember a lot discussion I had with people a while back (I’m not exactly sure what I’m doing) but as I understand it, there’s no concept of a class. It’s just an object, and instances of those objects are clones of the original, right?

But what is the exact purpose of this .prototype property in JavaScript? How does it relate to instantiating objects?


These slides really helped a lot to understand this topic.

Every JavaScript object has an internal property called [[Prototype]]. If you look up a property via obj.propName or obj['propName'] and the object does not have such a property – which can be checked via obj.hasOwnProperty('propName') – the runtime looks up the property in the object referenced by [[Prototype]] instead. If the prototype-object also doesn’t have such a property, its prototype is checked in turn, thus walking the original object’s prototype-chain until a match is found or its end is reached.

Some JavaScript implementations allow direct access to the [[Prototype]] property, eg via a non-standard property named __proto__. In general, it’s only possible to set an object’s prototype during object creation: If you create a new object via new Func(), the object’s [[Prototype]] property will be set to the object referenced by Func.prototype.

This allows to simulate classes in JavaScript, although JavaScript’s inheritance system is – as we have seen – prototypical, and not class-based:

Just think of constructor functions as classes and the properties of the prototype (ie of the object referenced by the constructor function’s prototype property) as shared members, ie members which are the same for each instance. In class-based systems, methods are implemented the same way for each instance, so methods are normally added to the prototype, whereas an object’s fields are instance-specific and therefore added to the object itself during construction.