Java development 2.0: JavaScript for Java developers

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Listing 8. Ruby doesn't play that way

> array = ["A", "B"]
> ans = 2 * array


The Ruby code in Listing 8 would error out with:

TypeError: Array
can't be coerced into Fixnum

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And if I tried to add "A" to array, it would yield:

TypeError: can't convert
Array into String


If I tried the same trick in Groovy, I would get something like this:

groovy.lang.MissingMethodException: No signature of method:
java.lang.Integer.plus() is applicable for argument types: (java.util.ArrayList) values:
[[A, B]]


Thus, you can see the various levels of weak typing in action. Clearly, if there was a scale for type weakness, JavaScript would be the wimpiest of them all!

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JavaScript functions

A JavaScript function, like a Java method, is a construct for defining and encapsulating reusable behavior. Functions in JavaScript look and feel at first glance a lot like Groovy's closures. Functions in JavaScript are objects. In fact, they are first-class objects, very much unlike methods in Java code. Because JavaScript functions are objects, they can be passed around to other functions and can be invoked at will.

Functions are defined with the function keyword. You can specify arguments like you would in a method declaration in the Java language, plus you can return something from a JavaScript function. Unlike dynamic languages, like Groovy or Ruby, where a return call is optional (and thus the last line of any method is returned), functions in JavaScript must use a return statement if they want to return something; otherwise, they return nothing.

You call functions in JavaScript like you would call closures in Groovy. In Listing 9, I define a simple function that takes no parameters. Its purpose is to print "blah" to the JavaScript console in Chrome.

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Listing 9. Defining and calling a function in JavaScript

> function blah() { console.log("blah"); }
> blah() //prints blah
> blah.call() //prints blah
> blah.apply() //prints blah


You can invoke a function directly with parentheses (that is, ()), via the call method, or via the apply method. The first class-ness of function objects is clearly demonstrated here. Listing 10 shows what happens when I try calling a garbage method on function blah:

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Listing 10. Functions as objects in JavaScript

> blah.foo()


In this case, you should see an error message complaining that foo isn't a method defined, like so:

TypeError: Object function blah() { console.log("blah"); } has no method 'foo'


Now read that error message again. It's complaining that foo isn't defined, which implies that if it was defined, things should have worked.

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Classes in JavaScript

JavaScript supports primitives, which I've discussed. It also supports objects, like Array. JavaScript doesn't support classes — at least not in the classical Java language sense. Because JavaScript is a prototype-based language, you don't define classes: instead, behavior is reused via cloning existing objects. Thus, in JavaScript, you don't define class objects, you define them in functions, then use nested functions to define behavior — something you've already seen in action.

To emulate a class, you define a function. You can give it a name (that is, a class name), specify parameters (as in a constructor), and even use the .this keyword, which basically means referencing a variable within the scope of the function. What's more, inner functions can be aliased to look like method calls.

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To demonstrate, in Listing 11 I'll create a Message prototype (also known as a class) that is super simple. I'll provide a few parameters (who the message is from, who it is to, and the actual message) and the class will represent my message as JSON.

Listing 11. Functions as classes in JavaScript

function Message(to, from, msg){
this.to = to;
this.from = from;
this.msg = msg;

this.asJSON = function(){
  return "{'to':'" + this.to + "', 'from':'" + this.from + "', 'message':'" +
    this.msg + "'}";
}
}

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In Listing 11, I defined a Message function — an object with a name and a few properties; namely, to, from, and msg. I then defined a property (asJSON) pointing to an inner function whose job it is to hand-jam a string representation of my JSON message.

Note that I can also define this "class" in a web page, load it with Chrome, open up the JavaScript console, and use it interactively. That's what's happening in Listing 12:

Listing 12. Using classes in JavaScript

> var message = new Message('Andy', 'Joe', 'Party tonight!');
> message.asJSON();
"{'to':'Andy', 'from':'Joe', 'message':'Party tonight!'}"

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This code looks almost like Groovy or even Java code (if not for the var), doesn't it? The fact is, it's totally possible to use OOP techniques for building JavaScript applications (that is, programs containing objects with data and methods interacting with each other).

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In conclusion

I hope this article has discredited the notion, now long outdated, of JavaScript as a toy language. In fact it's quite powerful and has a lot of the syntactic sugar and ease of newer languages like Groovy and Ruby. Some of the very qualities that made JavaScript suspect in the '90s are considered desirable today.

Given the importance of the web to much Java application development and the unique place JavaScript holds as a browser-compatible language, every Java programmer should be conversant in JavaScript — period. The browser (whether it be on a computer or mobile device, phone, or tablet) is the means by which more and more people interact with applications. JavaScript is also the common medium between all server-side languages. Besides, understanding some of JavaScript's features will help make you a better programmer in any language, including the one you call home.