I have just written a JavaScript file which calculates a taxi fare based on a starting zone, an ending zone, and the length of the taxi ride. The program appears to work fine, however, I would like to be able to display some type of alert or error message though if the proper input is not typed into the prompts. For example, there are only 3 zones available in my program, zones 1,2 and 3. If someone enters a 4 in the user prompt for the zone, I would like to be able to have a message appear indicating that zone 4 is not a valid zone. OR the alert of error message can even come up after the prompts when my HTML page loads completely stating an invalid character has been entered, please refresh the page and try again or something along those lines. If anyone has any suggestions or advice it would be greatly appreciated. I don't even need for anyone to give me the exact code, but just a direction to go into so I can figure it out. Thanks in advance and have a great day.

Your problem description is quite specific - but ultimately what you are talking about is some type of form validation.

Generally form validation falls into two categories:

1. things you can validate purely on the client - because you don't need access to data or logic on the server side,

and

2. things you can only validate on the server - by accessing server data and/or logic.

I don't know which yours falls into. If you have all the information on the page - then you can easily use Javascript for #1. If your problem falls into "requiring server data" - then you can still use JavaScript - but you will need to make an AJAX call... or otherwise you will have to do the validation on the server. Many server side frameworks include form validation as a built i concept in the framework. (such as Struts)