Object Attributes

Every object has associated prototype, class, and extensible attributes. The subsections that follow explain what these attributes do and (where possible) how to query and set them.

The prototype Attribute

An object’s prototype attribute specifies the object from which it inherits properties. (Review Prototypes and Inheritance for more on prototypes and property inheritance.) This is such an important attribute that we’ll usually simply say “the prototype of o” rather than “the prototype attribute of o.” Also, it is important to understand that when prototype appears in code font, it refers to an ordinary object property, not to the prototype attribute.

The prototype attribute is set when an object is created. Recall from Prototypes that objects created from object literals use Object.prototype as their prototype. Objects created with new use the value of the prototype property of their constructor function as their prototype. And objects created with Object.create() use the first argument to that function (which may be null) as their prototype.

In ECMAScript 5, you can query the prototype of any object by passing that object to Object.getPrototypeOf(). There is no equivalent function in ECMAScript 3, but it is often possible to determine the prototype of an object o using the expression o.constructor.prototype. Objects created with a new expression usually inherit a constructor property that refers to the constructor function used to create the object. And, as described ...

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