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6.8.1 The prototype Attribute

6.8.1 The prototype Attribute


An object’s prototype attribute specifies the object from which it inherits properties. (Review §6.1.3 and §6.2.2 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 §6.1.3 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 above, constructor functions have a prototypeproperty that specifies the prototype for objects created using that constructor. This is explained in more detail in §9.2 , which also explains why it is not a completely reliable method for determining an object’s prototype. Note that objects created by object literals or by Object.create() have a constructor property that refers to the Object() constructor. Thus, constructor.prototyperefers to the correct prototype for object literals, but does not usually do so for objects created with Object.create().

To determine whether one object is the prototype of (or is part of the prototype chain of) another object, use the isPrototypeOf() method. To find out if p is the prototype of o write p.isPrototypeOf(o). For example:

var p = {x:1}; // Define a prototype object. var o = Object.create(p); // Create an object with that prototype. p.isPrototypeOf(o) // => true: o inherits from p Object.prototype.isPrototypeOf(o) // => true: p inherits from Object.prototype

Note that isPrototypeOf() performs a function similar to the instanceof operator (see §4.9.4 ).

Mozilla’s implementation of JavaScript has (since the early days of Netscape) exposed the prototype attribute through the specially named __proto__ property, and you can use this property to directly query or set the prototype of any object. Using __proto__

6.8 Object Attributes | 135

is not portable: it has not been (and probably never will be) implemented by IE or Opera, although it is currently supported by Safari and Chrome. Versions of Firefox that implement ECMAScript 5 still support __proto__, but restrict its ability to change the prototype of nonextensible objects.

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