Core JavaScript 1.5 Guide:Loop Statements:for Statement

From MDC

[edit] for Statement

A for loop repeats until a specified condition evaluates to false. The JavaScript for loop is similar to the Java and C for loop. A for statement looks as follows:

for ([initialExpression]; [condition]; [incrementExpression])
   statement

When a for loop executes, the following occurs:

1. The initializing expression initialExpression, if any, is executed. This expression usually initializes one or more loop counters, but the syntax allows an expression of any degree of complexity. This expression can also declare variables.
2. The condition expression is evaluated. If the value of condition is true, the loop statements execute. If the value of condition is false, the for loop terminates. If the condition expression is omitted entirely, the condition is assumed to be true.
3. The statement executes. To execute multiple statements, use a block statement ({ ... }) to group those statements.
4. The update expression incrementExpression, if there is one, executes, and control returns to step 2.

Example
The following function contains a for statement that counts the number of selected options in a scrolling list (a Select object that allows multiple selections). The for statement declares the variable i and initializes it to zero. It checks that i is less than the number of options in the Select object, performs the succeeding if statement, and increments i by one after each pass through the loop.

<script type="text/javascript">//<![CDATA[

function howMany(selectObject) {
   var numberSelected = 0;
   for (var i = 0; i < selectObject.options.length; i++) {
      if (selectObject.options[i].selected)
         numberSelected++;
   }
   return numberSelected;
}

//]]></script>
<form name="selectForm">
   <p>
      <strong>Choose some music types, then click the button below:</strong>
      <br/>
      <select name="musicTypes" multiple="multiple">
         <option selected="selected">R&B</option>
         <option>Jazz</option>
         <option>Blues</option>
         <option>New Age</option>
         <option>Classical</option>
         <option>Opera</option>
      </select>
   </p>
   <p>
      <input type="button" value="How many are selected?"
         onclick="alert ('Number of options selected: ' + howMany(document.selectForm.musicTypes))"/>
   </p>
</form>