Reject unrecognised parameters

TL;DR: your API or library should error if it is passed parameters it does not recognise.

I recently spent a week debugging a friend’s JavaScript code. A datepicker library seemingly wasn’t picking up changes to the date format, supplied through options like so:

\$("#datepicker").datepicker({
"dateFormat": "yy-mm-dd",
});

We arrived at the solution after 43 emails – the code was using bootstrap-datepicker while the options were from jQueryUI-datapicker. We should have been using format rather than dateFormat. Both libraries use the same method, leading to a confusing half–working scenario.

Had the library failed noisily on unexpected parameters, the error would have been picked up almost immediately.

Your API or library should return errors or fail noisily if it is passed unexpected arguments. This helps your users understand that they are doing something wrong and lets them know how to fix it.