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If you were to store 5 pair of shoes, you might have a box for each pair. On each box you would write what is in it.
- The boxes would be your variables.
- Places to store things. - The name on the boxes would be the variable names.
- The ones you'd use when referring to each of the boxes. - And finally the shoes, would be the content of the variables.
- What is stored in the boxes.
A variable is simply a place in the computer's memory to store information. All variables are referred to by the unique name you assigned to them.
Consider this example:
html>
<head>
<title>My Javascript Page</title>
</head>
<body>
<script>
myname="Henrik";
document.write(myname);
</script>
</body>
</html>
<head>
<title>My Javascript Page</title>
</head>
<body>
<script>
myname="Henrik";
document.write(myname);
</script>
</body>
</html>
This example would write "Henrik" in the document.
Note that when you want text to be stored in a variable you need to put the text in " ".
The reason is that javascript uses " " to tell the difference between text and variables.
Look at this example to see the importance of this.
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Try to predict the output of the example before reading on.
- In the first line, the text "my first name" is stored in the Henrik variable.
- In the second line, the Henrik variable is stored in the myname variable.
- Finally in line 3, the myname variable is written to the document.
The result is that "my first name" will be written on the page.
ASSIGNING VALUES TO VARIABLES
The most common way to assign a value to a variable is using the equals sign.
Consider these examples to see the different ways variables can be assigned to contain either values or text.
Note in particular how parentheses can be used to control how complex formulas are handled.
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ARITHMETHIC OPERATORS
The above table includes the so-called "arithmethic operators" a++ and a--.
You could really live well without these, since what they do can be achieved by using the other operators available.
However you will often see them used in scripts, and you might even be lazy enough to use them yourself, since it is faster to type a++; than it is to type a=a+1;.
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COMPARING VARIABLES
There are several different ways to compare variables.
The simplest is comparing for equality, which is done using a double equals sign:
if (a==b) {alert("a equals b")};
if (lastname=="Petersen") {alert("Nice name!!!")};
Note: The if statement is explained in the next section.
If you forget to use double equals signs when comparing variables for equality, and use a single equals sign instead, you will not compare the variables. What will happen is that the variable on the left side of the equals sign will be assigned the value of the variable to the right.
An example of the error:
if (lastname="Petersen") {alert("Nice name!!!")};
This is a very common bug that will totally ruin the script.
This table contains the different comparing operators:
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July 29, 2007, 11:02 pm
Just to let you know, the increment/decrement opertors can have unexpected effects. They can be used as prefix/postfix operators, and this is where the unexpected effects come in. Consider these:
a = 5;
b = ++a;
c = a++;
What are the end values?
a = 7;
b = 6;
c = 6;
This is because when used as a prefix operator, it will increment/decrement, then ***ign the value, but as a postfix operator, it will ***ign the value, then increment/decrement.