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simply by changing the property, we may choose to implement this by hiding the actual property that describes status and use a public method such as updateStatus to allow us to change the way the status is internally represented within the object The OO principal of encapsulation defines the technique of hiding some properties and methods from the outside world while exposing others In many languages this is accomplished through the use of a public and private keyword, which designates members within the object as being hidden or visible JavaScript does not implement public and private directly but does provide a technique for hygiene properties and methods within an object In other words, it is possible to have functions defined within an object that are only callable by other functions within the object and not by external calls to the object Douglas Crockford provides an excellent description of how to do this on his web site:
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Because JavaScript objects are mutable (modifiable), new properties and methods can be added to the object at any time simply by referencing a member and giving it a value Our first technique for creating objects is to create a new object using the new keyword and then to assign various properties using the dot notation
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o1 = new Object(); o1counter = 0; o1incrementCounter = function() {thiscounter++;}
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The code here creates a new object containing a single property called counter, which has a starting value of numeric 0The object also contains a function called incrementCounter, which adds 1 to the current value of counter The problem with this approach is that each time we create a new instance of an object type, all the properties and methods must be explicitly assigned If there are any default values, they must be set All of the members must be defined, and there is no way to use existing definitions for these properties and methodsThis is a fairly unsatisfactory approach for most object oriented developers Given the size and complexity of most applications today, it is very useful to be able to have a standard template for what a new instance of an object should look likeThis provides for an important level of reuse necessary to ensure our productivity
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There are two primary techniques for providing object templatesThe first technique is to define the template explicitly as its own entity A blueprint for how to build the object is created in its own separate file as a class definitionThis option is utilized by the Java programming language by allowing developers to create class definitions in their own java filesThese template blueprints are very much like the blueprint for a house When you desire to build a new house, you create the house from an existing blueprint There s no limit to how houses you may create from the same blueprint, just as there is no limit to how many objects may be created from the class definition
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Just about every object oriented programming language follows an approach similar to that used by Java JavaScript is one of the few exceptions
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In Java, after the new object is created from its class definition, the properties and methods defined in that object cannot change No properties can be added or taken away, and no new methods can be defined or removedThe values of the properties can be changed, of course, but not the existence of the properties In Java, class definition files are compiled and included with the application at runtime, and they can be used over and over again to create as many instances of an object as the application programmer desires JavaScript is not constrained by limitation imposed in Java JavaScript does not use the class definition technique JavaScript uses an entirely different approach known as a prototype Rather than creating a new house from an existing blueprint, an existing house is copied to build a new one JavaScript objects are built from existing JavaScript objects You can think of a prototype as acting as the model for the new object While the analogy to building objects in Java is like building houses from blueprints, a better analogy for JavaScript is the creation of new objects by copying existing objects If you copy a 20-page document, your new document will also be 20 pages long, and it will in turn contain all the text and the typos of the original documentTo use the prototype technique for creating new objects, it is necessary to have an existing object containing the properties and methods that you want the new object to possess Although the prototype process is the technique that JavaScript uses to build new objects, it doesn t implement it as directly as you might expect For instance, there is no clone or build from function or keyword in JavaScript that allows you to directly build a new object from an existing object Rather the technique is a little more circuitousTo define a prototype to be used as a model for creating new objects, an additional object called the constructor must also be definedThe constructor is a function used to build and initialize new instances of an object for a given data typeThe constructor function is called when the new keyword is used in JavaScriptThe constructor function can contain assignments for the properties and methods of the new object There are still a few problems with this approach By defining the functions within the constructor, we are repeating code for the functions in every instance of an object that is created from the constructor Because functions can be rather large and because there is duplication of the exact same function code in each object, this approach provides us with objects that are much larger than necessary And if we desire to remove a function from each instance of the object or to change the function, it would be necessary to find all the existing objects of that datatype and to explicitly manipulate each one to either remove or change the functionWe would also be missing out on one of our key pillars of object oriented programming: inheritance Inheritance is the ability for an object of one type to inherit properties and methods from an object of their type
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