Validating Input Against Defined List?

I have a selection list that users can choose one value from about 300 different values. I wish to add some cross value vaidations to ensure some extra data gets entered should the user choose one of 50 of these values. Now I know I can write a JS validation individully fo each of the 50 as follows:

if theForm.field1.value=="25" && theorm.field2.value =="" {
alert("Enter field2 value");
}

Then repeat this alidation 50 times.

My question - is there a way to shorten the code to someting like this:

var fieldx = "1, 2, 3, 4, 5"; etc with all 50 values defined here, then something like:

if theForm.field1.value==fieldx && theForm.field2.value =="" {
alert("Enter field2 value");
}