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B.1 JavaScript is not Java

What’s in a name? In the case of Java and JavaScript, a lot of marketing and relatively little substance. JavaScript was renamed from “livescript” at the last minute by Netscape’s marketing department, and now the name has stuck. Contrary to popular perception, JavaScript is not a descendent of the C family of languages. It owes a lot more to functional languages such as Scheme and Self, and it has quite a lot in common with Python, too. Unfortunately, it’s been named after Java and syntactically styled to look like Java. In places, it will behave like Java, but in many places, it just plain won’t.

Table B.1 summarizes the key differences.

Table B.1 Key features of JavaScript and their implications

Feature

Implications

Variables are loosely typed.

Variables are just declared as variables, not as integers, strings, or objects of a specific class. In JavaScript, it is legal to assign values of different types to the same variable.

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Table B.1 Key features of JavaScript and their implications (continued) Feature

Implications

Code is dynamically interpreted.

At runtime, code is stored as text and interpreted into machine

instructions as the program runs, in contrast to precompiled languages such as Java, C, and C#. Users of your website can generally see the source code of your Ajax application. Furthermore, it allows for the possibility of code being generated dynamically by

other code without resorting to special bytecode generators.

JavaScript functions are

A Java object’s methods are tied to the object that owns them and

first-class objects.

can be invoked only via that object. JavaScript functions can be

attached to objects so that they behave like methods, but they

can also be invoked in other contexts and/or reattached to other

objects at runtime.

JavaScript objects are

A Java, C++, or C# object has a defined type, with superclasses

prototype-based.

and virtual superclasses or interfaces. This strictly defines its functionality. Any JavaScript object is just an object, which is just an associative array in disguise. Prototypes can be used to emulate

Java-style types in JavaScript, but the similarity is only skin deep. These differences allow the language to be used in different ways and open up the possibility of a number of weird tricks worthy of a seasoned Lisp hacker. If you’re a really clever, disciplined coder, you can take advantage of these tricks to do marvelous things, and you might even do so beyond a few hundred lines of code. If, on the other hand, you only think you’re really clever and disciplined, you can quickly end up flat on your face.

I’ve tried it a few times and come to the conclusion that keeping things simple is generally a good thing. If you’re working with a team, coding standards or guidelines should address these issues if the technical manager feels it is appropriate. However, there is a second reason for knowing about these differences and tricks: the browser will use some of them internally, so understanding what is going on can save you much time and pain in debugging a badly behaved application. In particular, I’ve found it helpful to know where the code is not behaving like a Java object would, given that much of the apparent similarity is only apparent. So read on, and find out what JavaScript objects really look like when the lights are out, how they are composed of member fields and functions, and what a JavaScript function is really capable of.

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APPENDIX B

JavaScript for object-oriented programmers

B.2 Objects in JavaScript

JavaScript doesn’t require the use of objects or even functions. It is possible to write a JavaScript program as a single stream of text that is executed directly as it is read by the interpreter. As a program gets bigger, though, functions and objects