typedef-converter

Typescript to Flow converter

This projects exists because having to write duplicate library definitions is no fun at all.

The state of the converter

It's surprisingly robust and non-lossy as it stands right now, in big part thanks to how similar flow and typescript definition files are. I've ran it against the typescript definition for yargs and it converted it to a flow library definition that worked out of the box.

It almost works with multiple files - biggest issue right now is not inserting the proper module, instead favouring the root module which should never have properties.

The difficult parts

Namespaces

Namespaces have been a big headache. What it does right now is that it converts any namespace to a module
and then imports any references to that module. What's currently not working in terms of namespaces is exporting all properties of the namespace as a default object, but that should be a fairly trivial change.

Variables

Since TS and flow variables dont match in functionality, the converter has to resolve variable references manually. A common case looks like this:

var yargs: yargs.Argv;
	export = yargs;

Which then resolves to declare module.exports: Argv.

External library imports

Definitions in TS and flow are often quite different, and imported types from other libraries dont usually have a one-to-one mapping. As an example, libraries using React.Component<> are difficult to translate to flow. This might require manual processing, or we add a set of hardcoded mutations that handle common cases.

Odd TS conventions

Lodash has been one of the reference libraries i've worked with when creating the converter. The definition is mostly just a series of interfaces with the same name being re-declared over and over again for each function, which doesn't translate to flow at all. If anyone knows how to make sense of the lodash definition, send me a tweet.

Usage

Standard usage (will produce lodash.flow.js):

yarn global add typedef-converter
typedef-converter lodash.d.ts

Options

-o / --out: Specifies the filename of the exported file