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6.2.2 Inheritance

6.2.2 Inheritance


JavaScript objects have a set of “own properties,” and they also inherit a set of properties from their prototype object. To understand this, we must consider property access in more detail. The examples in this section use the inherit() function from Exam ple 6-1 in order to create objects with specified prototypes.

Suppose you query the property x in the object o. If o does not have an own property with that name, the prototype object of o is queried for the property x. If the prototype object does not have an own property by that name, but has a prototype itself, the query is performed on the prototype of the prototype. This continues until the property x is found or until an object with a null prototype is searched. As you can see, the prototype attribute of an object creates a chain or linked list from which properties are inherited.

var o = {} // o inherits object methods from Object.prototype

o.x = 1; // and has an own property x. var p = inherit(o); // p inherits properties from o and Object.prototype

p.y = 2; // and has an own property y. var q = inherit(p); // q inherits properties from p, o, and Object.prototype

q.z = 3; // and has an own property z. var s = q.toString(); // toString is inherited from Object.prototype

q.x + q.y // => 3: x and y are inherited from o and p

Now suppose you assign to the property x of the object o. If o already has an own (noninherited) property named x, then the assignment simply changes the value of this existing property. Otherwise, the assignment creates a new property named x on the object o. If o previously inherited the property x, that inherited property is now hidden by the newly created own property with the same name.

Property assignment examines the prototype chain to determine whether the assignment is allowed. If o inherits a read-only property named x, for example, then the assignment is not allowed. (Details about when a property may be set are in §6.2.3 .) If the assignment is allowed, however, it always creates or sets a property in the original object and never modifies the prototype chain. The fact that inheritance occurs when querying properties but not when setting them is a key feature of JavaScript because it allows us to selectively override inherited properties:

var unitcircle = { r:1 }; // An object to inherit from var c = inherit(unitcircle); // c inherits the property r

c.x = 1; c.y = 1; // c defines two properties of its own

c.r = 2; // c overrides its inherited property unitcircle.r; // => 1: the prototype object is not affected

There is one exception to the rule that a property assignment either fails or creates or sets a property in the original object. If o inherits the property x, and that property is an accessor property with a setter method (see §6.6 ), then that setter method is called rather than creating a new property x in o. Note, however, that the setter method is called on the object o, not on the prototype object that defines the property, so if the setter method defines any properties, it will do so on o, and it will again leave the prototype chain unmodified.

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