Generating typescript declaration files from javascript

Say for example, you have a npm library, in my case mongoose, how would you go about generating d.ts files?

JavaScript doesn't always contain enough type information for the TypeScript compiler to infer the structures in your code - so automatically generating a definition based on JavaScript is rarely an option.

There are instructions on how to write them from scratch here:

https://www.stevefenton.co.uk/2013/01/complex-typescript-definitions-made-easy/

But there is one trick that might work (it only works in a limited set of cases).

If you paste the JavaScript into a new TypeScript file, fix any trivial errors you may get and compile it using the definition flag, it may be able to get you a file that would at least be a starting point.

tsc --declaration js.ts

The question is a bit old, but for the sake of people coming from search engines like me, if you are looking for an automated tool, check out:

• Microsoft/dts-gen

  The official starting point Microsoft uses when creating types. It's meant to be a starting point only though and I didn't get a lot of luck depending just on it

• dtsmake

  This one looks very promising. It depends on Ternjs which some editors use to provide autocomplete for JS code. Check Ternjs out also for other tool names and comparisons with them.

Update (2021): It's a good idea to check npmtrends and compare dts-generator, dts-gen, dtsmake, npm-dts, and alternatives.