Use the stuff you already have to create as documentation.
a. Write your specs as if for developers and users. Use the present tense, and state what your thing does.
"This npm module zumbinates nonrecursive framisses. It takes immutable framisses as input and generates immutable output."
See how much specification is there in just two sentences? See how useful it is to the user or the next dev.
b. Put some comments in your unit tests.