Script validator to ensure if project = ABC, sprint field is NOT empty?

Question

Hi, 
I'm making a simple script validator with Script-Runner on a workflow transition, and I want to ensure if project "ABC" or "DEF" is selected then the Sprint field should not be empty.

I can't find documentation on how to write something like this.
Could someone help me out with this? 
Thanks.

JamieA · Accepted Answer

if (issue.projectObject.key in ["ABC", "DEF"]) {
    return cfValues["Sprint"]
}
else {
    return true
}