Syntax Rules
Statements may contain a semicolon at the end of them as in C, but it is not necessary unless there are multiple statements on one line. Multiple statements on one line must be separated by a semicolon.
JavaScript is case sensitive.
Quotes may be single quote (') or double quote ("). When embedded within each other they must be used consistently as in the following example.
onMouseOver="window.status='To Operating Systems Section' ;return true"
Comments are the same as in C++ with the "//" characters for a single line comment, and the "/*" for the beginning of a multiline comment and the "*/" for the end of a multiline comment.
Variables must be defined before they are used.
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JavaScript Syntax Rules
JavaScript is a simple language, but you do need to be careful to use its syntax—the rules that define how you use the language—correctly. The rest of this book covers many aspects of JavaScript syntax, but there are a few basic rules you should understand to avoid errors.
Case Sensitivity
Almost everything in JavaScript is case sensitive: you cannot use lowercase and capital letters interchangeably. Here are a few general rules:
JavaScript keywords, such as for and if, are always lowercase.
Built-in objects such as Math and Date are capitalized.
DOM object names are usually lowercase, but their methods are often a combination of capitals and lowercase. Usually capitals are used for all but the first word, as in toLowerCase and getElementById.
When in doubt, follow the exact case used in this book or another JavaScript reference. If you use the wrong case, the browser will usually display an error message.
Variable, Object, and Function Names
When you define your own variables, objects, or functions, you can choose their names. Names can include uppercase letters, lowercase letters, numbers, and the underscore (_) character. Names must begin with a letter or underscore.
You can choose whether to use capitals or lowercase in your variable names, but remember that JavaScript is case sensitive: score, Score, and SCORE would be considered three different variables. Be sure to use the same name each time you refer to a variable.
Reserved Words
One more rule for variable names—they must not be reserved words. These include the words that make up the JavaScript language, such as if and for, DOM object names such as window and document, and built-in object names such as Math and Date. A complete list of reserved words is included in Appendix D, "JavaScript Quick Reference."
Spacing
Blank space (known as whitespace by programmers) is ignored by JavaScript. You can include spaces and tabs within a line, or blank lines, without causing an error. Blank space often makes the script more readable.
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