Yep, that's a great way to use Object.assign, but that doesn't really cover the variety of cases that prompted me to write this helper. For example, your code only works if options does not have a key for the default value, but not when it has a key with a null, undefined, or otherwise unacceptable value. const defaults = { name: '' }; const opts = { name: null }; Name would be set to null, which is a common case I try to avoid.
If someone explicitly passes in "const opts = { name: null }" then I would argue that he wants name to be null and it should not be overwritten by what is defined in defaults. But I agree with you that there may be some (rare?) cases where some other behaviour is wanted.
Well, I can't speak for the needs of every other developer or application, but I have personally encountered plenty of cases where null and undefined checks are necessary to avoid runtime errors.