Chapter 2: How to code a JavaScript application

This chapter presents a subset of JavaScript and DOM scripting that will soon have you writing significant applications. If you don't have any programming experience, this chapter also makes a great aptitude test. If you read it and can do the exercises at the end of the chapter, you're ready for the rest of this book.

How to create identifiers

Variables, functions, objects, properties, methods, and events must all have names so you can refer to them in your JavaScript code. An identifier is the name given to one of these components.

Figure 2-6 shows the rules for creating identifiers in JavaScript. Besides the first four rules, you can't use any of the JavaScript reserved words (also known as keywords) as an identifier. These are words that are reserved for use within the JavaScript language. You should also avoid using any of the JavaScript global properties or methods as identifiers. Although you can sometimes get away with this, it will lead to problems in some applications.

Besides the rules, you should give your identifiers meaningful names. That means that it should be easy to tell what an identifier refers to and easy to remember how to spell the name. To create names like that, you should avoid abbreviations. If, for example, you abbreviate the name for monthly investment as mon_inv, it will be hard to tell what it refers to and hard to remember how you spelled it. But if you spell it out as monthly_investment, both problems are solved.

Similarly, you should avoid abbreviations that are specific to one industry or field of study unless you are sure the abbreviation will be widely understood. For example, mpg is a common abbreviation for miles per gallon, but cpm could stand for any number of things and should be spelled out.

To create an identifier that has more than one word in it, most JavaScript programmers use a convention called camel casing. With this convention, the first letter of each word is uppercase except for the first word. For example, monthlyInvestment and taxRate are identifiers that use camel casing.

The alternative is to use underscore characters to separate the words in an identifier. For example, monthly_investment and tax_rate use this convention. If the standards in your shop specifies one of these conventions, by all means use it. Otherwise, you can use either convention. In either case, though, be consistent.

Rules for creating identifiers in JavaScript

• Identifiers can only contain letters, numbers, the underscore, and the dollar sign.
• Identifiers can't start with a number.
• Identifiers are case-sensitive.
• Identifiers can be any length.
• Identifiers can't be the same as reserved words.
• Avoid using global properties and methods as identifiers. If you use one of them, you won't be able to use the global property or method that the identifier refers to.
• Avoid using names that are similar to reserved words, global properties, or global methods. For example, avoid using "Switch" since it is similar to "switch".

Reserved words in JavaScript (words for future use are italicized)
abstract	else	instanceof	switch
boolean	enum	int	synchronized
break	export	interface	this
byte	extends	long	throw
case	false	native	throws
catch	final	new	transient
char	finally	null	true
class	float	package	try
const	for	private	typeof
continue	function	protected	var
debugger	goto	public	void
default	if	return	volatile
delete	implements	short	while
do	import	static	with
double	in	super

Global properties and methods in JavaScript
alert	focus	prompt
blur	getComputedStyle	resizeBy
clearInterval	Infinity	resizeTo
clearTimeout	isFinite	scroll
close	isNaN	scrollBy
confirm	moveBy	scrollTo
decodeURI	moveTo	setInterval
decodeURIComponents	NaN	setTimeout
encodeURI	open	undefined
encodeURIComponents	parseFloat	unescape
escape	parseInt
eval	print

Valid identifiers in JavaScript
subtotal	index_1	$
taxRate	calculate_click	$log

Description

• Identifiers are the names given to variables, functions, objects, properties, and methods.

Figure 2-6 How to create identifiers