babel-plugin-flow-to-typescript

[Babel] plugin to convert [Flow] code into [TypeScript]

How to use

$ npm install -g @babel/cli @babel/core
$ npm install babel-plugin-flow-to-typescript

# you must use babel@^7.x.x
$ babel --version
7.4.4 (@babel/core 7.4.5)

$ babel --plugins babel-plugin-flow-to-typescript ${SRC_FLOW_FILE} -o ${DEST_TS_FILE}

Implementation status

Supported?	Syntax	Flow	TypeScript
✅	Maybe type	let a:?number	let a: number | null | undefined
✅	Void type	void	void
✅	Object type	Object	object
✅	Mixed type	mixed	unknown
✅	Function type	(A, B) => C	(x1: A, x2: B) => C
✅	Exact type	{| a: A |}	{ a: A }
✅	Indexers	{ [A]: B }	{ [a: A]: B }
✅	Existential type	Map<*, *>	Map<any, any>
✅	Opaque types	opaque type A = B	type A = B
✅	Variance	interface A { +b: B, -c: C }	interface A { readonly b: B, c: C }
✅	Type parameter bounds	function f<A: string>(a:A){}	function f<A extends string>(a:A){}
✅	Cast	(a: A)	(a as A)
✅	type/typeof import	import type A from 'module'	import A from 'module'
✅	$Keys	$Keys<X>	keyof X
✅	$Values	$Values<X>	X[keyof X]
✅	$ReadOnly	$Readonly<X>	Readonly<X>
✅	$Exact	$Exact<X>	X
✅	$Diff	$Diff<X, Y>	Pick<X, Exclude<keyof X, keyof Y>>
✅	$PropertyType	$PropertyType<T, k>	T[k]
✅	$ElementType	$ElementType<T, k>	T[k]
✅	$Shape	$Shape<T>	Partial<T>
✅	Class	Class<T>	typeof T
✅	typeof operator	typeof foo	typeof foo
✅	JSX	-	-
✅	Tuple type	[number, string]	[number, string]
✅	Type alias	type A = string	type A = string
✅	Flow Ignore	$FlowFixMe	any
✅	Interfaces	interface X { +prop: string }	interface X { readonly prop: string }
✅	Optional Members	a?.b	...
✅	Declare functions	declare function x(false): true;	function x(x0: false): true;
✅	Declare Class	...	...

Additions by Culture Amp

• Improvements to conversion of record-like types
  • Detection and production of a typescript "mapped" type
  • Detection and fixing of a generic parameter that should be constrained to "string | number" - an index key type
  • Empty object variable declarator ⇒ Annotated with record type
    • const foo = {} ⇒ const foo: Record<string | number, unknown> = {} [babel]: https://github.com/babel/babel [flow]: https://github.com/facebook/flow [typescript]: https://github.com/Microsoft/TypeScript