Variables

A variable can be said to be a Global Variable if it is declared outside of the bounds of a function. It must exist on its own. In the declaration of a Global Variable, the programmer is making available the information contained within the variable to every function and statement within that document, as well as the functions and statements represented in an external JavaScript Library. It may be of any data type that will be used within the applicable functions.

It is entirely possible for two variables with different contents, or even the same contents, to exist at the same time providing one is a Local Variable and one is a Global Variable. This case is normally used when the contents of the variable will not change, and must be used within a function as a Local Variable, then outside of a function as a Global Variable.

Creating and Naming Variables

• The first character of a valid variable name must be a letter or an underscore. Numbers, punctuation marks and special characters are not tolerated under any circumstances.
• There can be any amount of alphanumeric characters following the first character, not including punctuation marks and special characters.
• All characters must conform to the basic definition of the ISO-Latin-1 character set.
• The letters used may be upper or lower case letters, although JavaScript does distinguish between the two cases. For example, the variable named variableOne is different in the eyes of JavaScript from the variable named variableOne. Case matters when referencing variable names.

1. var variableName, variableName, variableName; - You can see in this syntax example that there are three variables being declared at once. You may declare as many variables as you have a need for on one line. Keep in mind that the variables declared in this manor are without values assigned to them. They are empty, and merely exist until you stuff something into them.
2. var variableName = value; - This syntax example shows the creation of one variable. The variable has been assigned an initial value with the value addition to the end of the statement.
3. variableName; - This example shows the creation of a variable without the use of the var keyword. This is an entirely legal thing to do in JavaScript, and saves typing, albeit a small amount. It is recommended you use the var keyword with every variable declaration in order to curtail any problems you may have with variable scope definitions.
4. variableName = value; - This example shows the creation of a variable without the use of the var keyword. This is an entirely legal thing to do in JavaScript, and saves typing, albeit a small amount. It is recommended you use the var keyword with every variable declaration in order to curtail any problems you may have with variable scope definitions. You can see that an initial value has been assigned to the variable, which can be used at a later time.

You can see in the above list that the "var" keyword isn't always necessary - JavaScript is considered to be a "loosely typed" language. This means that there is a lot of flexibility in the way you write your code. Notice also that every way of declaring a variable ended with a semi-colon. This too isn't absolutely necessary for JavaScript. The JavaScript engine takes educated guesses concerning the omitting of semi-colons, and is usually correct in its assumptions - though not always. Ideally, you should remember to end every line of code with a semi-colon. It is a good programming convention that takes the guesswork out of execution by the JavaScript engine. It is recommended you use the semi-colons at the end of every line.

The below example shows the creation of four variables:

<SCRIPT LANGUAGE="JavaScript">
<! --

var globalOne = "This is ";
globalTwo = "a string";

function writeVariables() {
   var localOne = " of text";
   document.write(globalOne, globalTwo, localOne);
}

-- >
</SCRIPT> 

The example shows the creation of a variable called globalOne using the var keyword, the content of which is the string "This is ". It is of the String data type because it is enclosed within double quotes, as stated in an earlier section. Next comes another variable called globalTwo that doesn't use the var keyword, but is still a valid JavaScript variable. It is given the value of "a string" which again is a string value because of the nesting of the text within double quotes. Then comes the function, named writeVariables. In this function is the local variable fittingly called localOne. It too contains a string value -" of text". Notice that a space has been left at the front of the text. JavaScript will honor one space appearing before the contents of a string variable, but no more. That is, if you put one space within the text at the beginning of the string and tell the browser to write it, the browser will write that one space. But if you insert more spaces than one, JavaScript will disregard the rest. The document.write statement is then used to reference, then write the contents of the three variables to the browser screen (that's what a document.write statement does - more later). The output of the script looks as follows:

This is a string of text

There is more that you can do with variables, but for now this example is just to give you an idea of how the variable is declared, and a bit about the use of the variable. Almost all of the coming discussions on the JavaScript language involve the use of variables in one form or another, so you'll learn how to think in terms of variables as you go. You can see that there is nothing to declaring, naming, and working with variables. The makers of JavaScript have made it very easy on you, the budding web developer. Enjoy.

Stop by next week where we'll cover all the fun that arrays can be.

Data Types - Strings
JavaScript Introduction
The JavaScript Array

The JavaScript Chronicles