java qr code generator library B.2.5 Reflecting on JavaScript objects in Java

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B.2.5 Reflecting on JavaScript objects
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In the normal course of writing code, the programmer has a clear understanding of how the objects he is dealing with are composed, that is, what their properties and methods do. In some cases, though, we need to be able to deal with completely unknown objects and discover the nature of their properties and methods before dealing with them. For example, if we are writing a logging or debugging system, we may be required to handle arbitrary objects dumped on us from the outside world. This discovery process is known as reflection, and it should be familiar to most Java and .NET programmers. If we want to find out whether a JavaScript object supports a certain property or method, we can simply test for it:
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if (MyObject.someProperty){ ... }
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This will fail, however, if MyObject.someProperty has been assigned the boolean value false, or a numerical 0, or the special value null. A more rigorous test would be to write
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if (typeof(MyObject.someProperty) != "undefined"){
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APPENDIX B
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JavaScript for object-oriented programmers
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If we are concerned about the type of the property, we can also use the instanceof operator. This recognizes a few basic built-in types:
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if (myObj instanceof Array){ ... }else if (myObj instanceof Object){ ... }
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as well as any class definitions that we define ourselves through constructors:
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if (myObj instanceof MyObject){ ... }
If you do like using instanceof to test for custom classes, be aware of a couple of gotchas. First, JSON doesn t support it anything created with JSON is either a JavaScript Object or an Array. Second, built-in objects do support inheritance among themselves. Function and Array, for example, both inherit from Object, so the order of testing matters. If we write
function testType(myObj){ if (myObj instanceof Array){ alert("it's an array"); }else if (myObj instanceof Object){ alert("it's an object"); } } testType([1,2,3,4]);
and pass an Array through the code, we will be told correctly that we have an Array. If, on the other hand, we write
function testType(myObj){ if (myObj instanceof Object){ alert("it's an object"); }else if (myObj instanceof Array){ alert("it's an array"); } } testType([1,2,3,4]);
then we will be told that we have an Object, which is also technically correct but probably not what we intended. Finally, there are times when we may want to exhaustively discover all of an object s properties and functions. We can do this using the simple for loop:
function MyObject(){ this.color='red'; this.flavor='strawberry';
Objects in JavaScript
this.azimuth='45 degrees'; this.favoriteDog='collie'; } var myObj=new MyObject(); var debug="discovering...\n"; for (var i in myObj){ debug+=i+" -> "+myObj[i]+"\n"; } alert(debug);
This loop will execute four times, returning all the values set in the constructor. The for loop syntax works on built-in objects, too the simple debug loop above produces very big alert boxes when pointed at DOM nodes! A more developed version of this technique is used in the examples in chapters 5 and 6 to develop the recursive ObjectViewer user interface. There is one more feature of the conventional object-oriented language that we need to address the virtual class or interface. Let s look at that now.
B.2.6 Interfaces and duck typing
There are many times in software development when we will want to specify how something behaves without providing a concrete implementation. In the case of our Shape object being subclassed by squares, circles, and so on, for example, we know that we will never hold a shape in our hands that is not a specific type of shape. The base concept of the Shape object is a convenient abstraction of common properties, without a real-world equivalent. A C++ virtual class or a Java interface provides us with the necessary mechanism to define these concepts in code. We often speak of the interface defining a contract between the various components of the software. With the contract in place, the author of a Shape-processing library doesn t need to consider the specific implementations, and the author of a new implementation of Shape doesn t need to consider the internals of any library code or any other existing implementations of the interface. Interfaces provide good separation of concerns and underpin many design patterns. If we re using design patterns in Ajax, we want to use interfaces. JavaScript has no formal concept of an interface, so how do we do it The simplest approach is to define the contract informally and simply rely on the developers at each side of the interface to know what they are doing. Dave Thomas has given this approach the engaging name of duck typing if it walks like a duck and it quacks like a duck, then it is a duck. Similarly with our Shape interface, if it can compute an area and a perimeter, then it is a shape.
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