The JavaScript Diaries: Part 4

By Lee Underwood

Global and Local Variables

As you have seen, variables can be used within functions, but not all variables used within a function are the same. There are two types of variables used within a function: global and local.

Global Variables

These are variables declared outside the function, within the overall script. If the value of a global variable is reassigned (changed) within a function, it will be reassigned throughout the entire script. This is because it "belongs" to the script itself. When a global variable is sent to a function, just the variable name is used (don't use the reserved word var). All of our examples so far in this installment have been local since all used the var reserved word.

If we create the following script, and declare the variables using the reserved word var, it will write "I like the Delta blues" on the page:

<script type="text/javascript">
<!--
  var place="Delta";
  var type="blues";
  document.write("I like the " + place + " " + type);
//-->
</script>

If we take the same script, add a function and reuse the global variables above within the function without using the reserved word var, it will create a bit of a problem, as shown below. Put this in the <head> section:

<script type="text/javascript">
<!--
  var place="Delta";
  var type="blues";
  function music() {
    place="New Orleans";
    type="jazz"
    document.write("I like " + place + " " + type);
  }
//-->
</script>

Then, add this in the <body> section:

<script type="text/javascript">
<!--
  music();
  document.write(" but I really like " + place + " " + type);
//-->
</script>

Here is what will happen:

  • The variables place and type will be declared and initialized with the values of "Delta" and "music," respectively.
  • Next, the function music() is declared.
  • The next event is the function call, music();. (Remember, the function itself is merely loaded into memory. It's not executed until it's called.)
  • The variables place and type, which are located within the function music(), are then reassigned new values, "New Orleans" and "jazz," respectively. They are reassigned because they are global variables.
  • The script will then write to the Web page, "I like New Orleans and jazz but I really like New Orleans and jazz." The problem is that you reassigned the global variables place and type, which changed their value throughout the entire script.

Local Variables

These variables are declared within the function (hence, they are "local"). In order to be declared locally they must be declared using the reserved word var. You can even use the same name as a global variable but its value will only be used within the function. If we use our previous example and add the var reserved word before the variables within the function, we will get the anticipated result. Go ahead and try both of them yourself.

<script type="text/javascript">
<!--
  var place="Delta";
  var type="blues";
  function music()
  {
    var place="New Orleans";
    var type="jazz"
    document.write("I like " + place + " " + type);
  }
//-->
</script>
<script type="text/javascript">
<!--
  music();
  document.write(" but I really like the " + place + " " + type);
//-->
</script>

As far as the JavaScript interpreter sees it, the variables place and type declared at the very beginning of the script are completely different than the variables place and type declared locally within the function. Be sure to remember that, without using the var reserved word within the function when declaring variables, you will be assigning a new value to an existing global variable.




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