Last Updated: February 25, 2016
· steveniseki

JavaScript difference between __proto__ and prototype


__proto__ is the actual object that is used in the lookup chain to resolve methods. It is a property that all objects have. This is the property which is used by the JavaScript engine for inheritance. According to ECMA specifications it is supposed to be an internal property, however most vendors allow it to be accessed and modified.


prototype is a property belonging only to functions. It is used to build __proto__ when the function happens to be used as a constructor with the <i>new</i> keyword

In prototype-based object oriented languages like Self and Javascript, every object in the system has a field that says "if I don't have a property or method that is requested of me, go to the object that this field references my <i>prototype</i> and look for it". Since that object will also have this "prototype" field as well, this becomes a recursive process. It is what is meant by a <i>prototype chain.</i> Note that this means that in a prototype language, there is no abstract concept of a "class"

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