I am currently studying web design (with emphasis on the design part) and I'm trying to do a small bit of work on the side to beef up my portfolio. I believe this is a JavaScript question:
I have a client who would like a particular page to require that the user enter a code (there are many which are acceptable) from a voucher they have before they are transported to another page. I can't seem to find this function. If anyone knows about this any info would be GREATLY appreciated!

It's a function you will have to create.

Depending on how many valid codes there will be, it might be better to handle it server side (ASP ASP.NET PHP) where you could have the codes in a database.

It could be done client side provided there are not many codes as they would have to be embedded into the page code or if the codes have an identifiable pattern.

IF this meant to be secure(ish) or to provide discounts it MUST be done server side, otherwise anyone could view the source and find out the codes.