Naked JavaScript objects

I was looking through one of the source files of Narcissus, the JavaScript interpreter written in JavaScript, when I came across a line that I probably missed before:

var keywords = {__proto__: null};

The __proto__ property is only accessible in Firefox, Safari, Rhino, and ActionScript and is the property that ties an object instance to its prototype. A little-known fact about JavaScript is that object instances have no relationship to their constructors, only to their prototypes. The __proto__ property exposes this relationship.

As with most properties, __proto__ can be overwritten. Doing so changes the prototype chain of the object. The code from Narcisuss effectively creates a JavaScript object that has no prototype chain and therefore none of the methods that all objects inherit from Object. The result? A truly naked base object that has no properties. Cutting off the prototype chain also ensures that changes to Object.prototype won’t effect the use of for-in. This is really interesting:

var o = { __proto__: null };<br /> alert(o.toString); //undefined

Interestingly, this seems to be the only way to create a naked object. You can define a constructor whose prototype is set to null, but creating an instance using that constructor automatically resets the prototype to Object. Example:

function MyObject(){}<br /> MyObject.prototype = null;<br /> var o = new MyObject();<br /> alert(o.toString); //function

I don’t know that this information is useful in any way, but it certainly is interesting.

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