Javascript Basics

By: Nicholas C. Zakas

ECMAScript is the standard that set the basics of Javascript syntax. Developers familiar with languages such as Java, C, and Perl will find ECMAScript syntax easy to pick up because it borrows syntax from each. Java and ECMAScript have several key syntax features in common, as well as some that are completely different.

The basic concepts of ECMAScript are the following:

Everything is case-sensitive. Just as with Java, variables, function names, operators, and everything else is case-sensitive, meaning that a variable named test is different from one named Test.
Variables are loosely typed. Unlike Java and C, variables in ECMAScript are not given a specific type. Instead, each variable is defined using the var operator and can be initialized with any value. This enables you to change the type of data a variable contains at any point in time (although you should avoid doing so whenever possible). Some examples:

var color = “red”;

var num = 25;

var visible = true;

End-of-line semicolons are optional. Java, C, and Perl require that every line end with a semicolon (;) to be syntactically correct; ECMAScript allows the developer to decide whether or not to end a line with a semicolon. If the semicolon is not provided, ECMAScript considers the end of the line as the end of the statement (similar to Visual Basic and VBScript), provided that this doesn’t break the semantics of the code. Proper coding practice is to always include the semicolons because some browsers won’t run properly without them, but according to the letter of the ECMAScript standard, both of the following lines are proper syntax:

var test1 = “red”

var test2 = “blue”;

Comments are the same as in Java, C, and Perl. ECMAScript borrowed its comments from these languages. There are two types of comments: single-line and multiline. The single-line comments begin with two forward-slashes (//), whereas multiline comments begin with a forward-slash and asterisk (/*) and end with an asterisk followed by a forward-slash (*/).

//this is a single-line comment

/* this is a multiline

comment */

Braces indicate code blocks. Another concept borrowed from Java is the code block. Code blocks are used to indicate a series of statements that should be executed in sequence and are indicated by enclosing the statements between an opening brace ({) and a closing brace (}).

For example:

if (test1 == “red”) {

test1 = “blue”;

alert(test1);

}

If you are interested in the specifics of ECMAScript’s grammar, The ECMAScript Language Specification (ECMA-262) is available for download from ECMA’s Web site, at www.ecma-international.org.