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1.21.0 • Public • Published


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. Please see the output in this flow-typed PR for the state of the output.

Supported? Syntax TypeScript Flow
Void type void void
Undefined type undefined void
Unknown type unknown mixed
Symbol type symbol Symbol
Unique symbol type unique symbol Symbol
Object type object {[key: string]: any}
Never type never empty
Variance interface A { readonly b: B, c: C } interface A { +b: B, c: C }
Functions (a: A, b: B) => C (a: A, b: B) => C
Indexers {[k: string]: string} {[k: string]: string}
This type (this: X, a: A, b: B) => C (a: A, b: B) => C
Type guards (a: X) => a is A (a: X) => boolean
Type parameter bounds function f<A extends string>(a:A){} function f<A: string>(a:A){}
keyof X keyof X $Keys<X>
X[keyof X] X[keyof X] $ElementType<X, $Keys<X>>
Partial Partial<X> $Rest<X, {}>
Readonly Readonly<X> $ReadOnly<X>
ReadonlyArray ReadonlyArray<X> $ReadOnlyArray<X>
ReadonlySet ReadonlySet<X> $ReadOnlySet<X>
ReadonlyMap ReadonlyMap<X, Y> $ReadOnlyMap<X, Y>
Record Record<K, T> { [key: K]: T }
Pick Pick<T, K>
Exclude Exclude<T, U>
Extract Extract<T, U>
NonNullable NonNullable<X> $NonMaybeType<X>
ReturnType ReturnType<F> $Call<<R>((...args: any[]) => R) => R, F>
InstanceType InstanceType<X>
Required Required<X>
ThisType ThisType<X>
T['string'] T['string'] $PropertyType<T, k>
T[k] T[k] $ElementType<T, k>
Mapped types {[K in keyof Obj]: Obj[K]} $ObjMapi<Obj, <K>(K) => $ElementType<Obj, K>>
Conditional types A extends B ? C : D any
typeof operator typeof foo typeof foo
Tuple type [number, string] [number, string]
Type alias type A = string type A = string
type/typeof import import A from 'module' import type A from 'module'


Install using npm i flowgen --save

import { compiler } from 'flowgen';

// To compile a d.ts file
const flowdef = compiler.compileDefinitionFile(filename);

// To compile a string
const flowdef = compiler.compileDefinitionString(str);

// To compile a typescript test file to JavaScript
// esTarget = ES5/ES6 etc
const testCase = compiler.compileTest(path, esTarget)

Recommended second step:

import { beautify } from 'flowgen';

// Make the definition human readable
const readableDef = beautify(generatedFlowdef);


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

npm i -g flowgen
flowgen lodash.d.ts


-o / --output-file [outputFile]: Specifies the filename of the exported file, defaults to export.flow.js

Flags for specific cases

--flow-typed-format: Format output so it fits in the flow-typed repo
--compile-tests: Compile any sibling <filename>-tests.ts files found
--no-inexact: Do not mark object types as inexact (using `...`)
--no-module-exports: Convert `export = Type` only to default export, instead of `declare module.exports: Type`
--interface-records: Convert TypeScript interfaces to Exact Objects
--no-jsdoc: Ignore TypeScript JSDoc
--add-flow-header: Add `// @flow` to generated files. Should be used for libs.

The difficult parts


Namespaces have been a big headache. What it does right now is that it splits any namespace out into prefixed global scope declarations instead. It works OK, but its not pretty and there's some drawbacks to it.

External library imports

Definitions in TS and flow are often quite different, and imported types from other libraries don't usually have a one-to-one mapping. Common cases are React.ReactElement, React.CSSPropsetc. 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. There's multiple ways of solving this but I don't have a great solution for it in place yet.

Sample of finding all typescript definition files and generate flow file with shell script

If your typescript definition files are built in lib add below shell script and run it.

for i in $(find lib -type f -name "*.d.ts");
  do sh -c "flowgen $i -o ${i%.*.*}.js.flow";

So if you have definition files in different dir, you can rename lib and run the script.

Here’s an example of the above as an npm script in package.json that excludes any typescript definition files found inside node_modules:

  "scripts": {
    "build:flowtypes": "find . -type f -not -path './node_modules/*' -name '*.d.ts' -exec sh -c 'yarn flowgen --add-flow-header $1 -o ${1%.*.*}.js.flow' _ '{}' \\;"

You can then have a build script that generates flow types along the lines of tsc --build && npm run build:flowtypes.


All help is appreciated. Please tweet at me if you want some help getting started, or just want to discuss ideas on how to solve the trickier parts.


  • git pull origin master
  • yarn compile
  • Change the version in package.json
  • git add .
  • `git commit -m "New release"
  • npm publish
  • git push




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  • joarwilk
  • orta