IndexOf returns the position in a data type or structure where the first occurence of a passed-in argument appears.
If no occurence is found, it returns "-1".
Before embracing regular expressions, I used this heavily for assessing strings.
However, a friend reminded me recently that this method works with arrays as well as strings**
With an array, the method returns the index of the array item where the value occurs.
In a string, it's the character position.
Here's the caveat...it doesn't appear to work in IE.
Here's a quick test to demonstrate how it works (or fails) in your browser of choice...