If statement if a value in a field changes

Need advice on how to write javascript based on the below logic.

I have a field that has a default value in it, and I want a result to occur based on if the default value in that field changes.

Field name: changeInContractDuration
Default value in field: 0 months

If anything other than 0 months appears in that field, the result changes to another field.

Make sense? Any advice appreciated!

What you are looking for is called an event listener. Look around for some information about event listeners and see how far that gets you.