JavaScript Prototype in Plain Language

»jan. 25 2013 147

Prototype is a fundamental concept that every JavaScript developer must understand, and this article aims to explain JavaScript’s prototype in plain, detailed language. If you don’t understand JavaScript’s prototype after reading this blog post, please ask questions in the comments below. I will personally answer all questions.

To understand prototype in JavaScript you must understand objects in JavaScript. If you aren’t already familiar with objects, you should read my post JavaScript Objects in Detail. Also, know that a property is simply a variable defined on a function.

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There are two interrelated concepts with prototype in JavaScript:

  1. First, every JavaScript function has a prototype property (this property is empty by default), and you attach properties and methods on this prototype property when you want to implement inheritance. This prototype property is not enumerable; that is, it isn’t accessible in a for/in loop. But Firefox and most versions of Safari and Chrome have a __proto__ “pseudo” property (an alternative syntax) that allows you to access an object’s prototype property. You will likely never use this __proto__ pseudo property, but you should know that it exists and it is simply a way to access an object’s prototype property in some browsers.

    The prototype property is used primarily for inheritance; you add methods and properties on a function’s prototype property to make those methods and properties available to instances of that function.

    Consider this simple example of inheritance with the prototype property (more on inheritance later):
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