Flow-to-TypeScript 
Compile Flow files to TypeScript
In Pre-Alpha - contributions welcome.
Installation
# Using Yarn:
yarn add flow-to-typescript
# Or, using NPM:
npm install flow-to-typescript --save
Usage
CLI
# Install globally
yarn global add flow-to-typescript
# Compile a file (all of these are equivalent)
flow2ts my/file.js.flow
flow2ts my/file.js.flow my/file.ts
flow2ts my/file.js.flow > my/file.ts
flow2ts -i my/file.js.flow -o my/file.ts
flow2ts --input my/file.js.flow --output my/file.ts
Programmatic
import { compile } from 'flow-to-typescript'
import { readFileSync, writeFileSync } from 'fs'
let path = 'path/to/file.js.flow'
let file = readFileSync(path, 'utf-8')
compile(file, path).then(ts =>
writeFileSync('path/to/file.ts', ts)
)
TypeScript vs. Flow
Features
| Done? | Flow | TypeScript | |
|---|---|---|---|
| ✅ | Maybe | ?type (NullableTypeAnnotation) |
type | null | undefined |
| ✅ | Null | null |
null |
| ✅ | Undefined | typeof undefined |
undefined |
| ✅ | Mixed | mixed |
unknown |
| ✅ | Void | void |
void |
| ✅ | Functions | (A, B) => C |
(a: A, b: B) => C |
| ⚔ | Predicates (0) | (a: A, b: B) => %checks |
(a: A, b: B) => C |
| ⚔ | Predicates (1) | (a: A, b: B) => C %checks |
(a: A, b: B) => C |
| ✅ | Exact types | {| a: A |} |
{ a: A } |
| ✅ | Indexers | { [A]: B } |
{ [a: A]: B } |
| ✅ | Opaque types | opaque type A = B |
type A = B (not expressible) |
| ✅ | Variance | interface A { +b: B, -c: C } |
interface A { readonly b: B, c: C } |
| ✅ | Bounds | <A: string> |
<A extends string> |
| ✅ | Casting | (a: A) |
(a as A) |
| ✅ | Import default type | import type A from './b' |
import A from './b' |
| ✅ | Import named type | import type { A } from './b' |
import { A } from './b' |
Utilities
| Done? | Flow | TypeScript | |
|---|---|---|---|
| Keys | $Keys<A> |
keyof A |
|
| Values | $Values<A> |
A[keyof A] |
|
| ✅ | ReadOnly | $ReadOnly<A> |
Readonly<A> |
| ✅ | Exact | $Exact<A> |
A |
| Difference | $Diff<A, B> |
TODO` | |
| Rest | $Rest<A, B> |
Exclude |
|
| Property type | $PropertyType<T, k> |
T[k] |
|
| Element type | $ElementType<T, K> |
T[k] |
|
| Dependent type | $ObjMap<T, F> |
TODO | |
| Mapped tuple | $TupleMap<T, F> |
TODO | |
| Return type | $Call<F> |
ReturnType |
|
| Class | Class<A> |
typeof A |
|
| Supertype | $Supertype<A> |
any (warn - vote for https://github.com/Microsoft/TypeScript/issues/14520) |
|
| Subtype | $Subtype<A> |
B extends A |
|
| Existential type | * |
any (warn - vote for https://github.com/Microsoft/TypeScript/issues/14466) |
✅ Done
⚔ Babylon doesn't support it (yet)
