How do you export a Flow type definition that is imported from another file?

Question

Given a type definition that is imported from another module, how do you re-export it?

/**
 * @flow
 */

import type * as ExampleType from './ExampleType';

...

// What is the syntax for exporting the type?
// export { ExampleType };

Instead of writing `import type * as ExampleType from './ExampleType';` you can write `import type ExampleType from './ExampleType';` — Gabe Levi, Sep 22 '15 at 21:49

score 33 · Accepted Answer · answered Sep 22 '15 at 21:49

33

The simplest form of this question is "how do I export a type alias?" and the simple answer is "with export type!"

For your example, you can write

/**
 * @flow
 */

import type * as ExampleType from './ExampleType';
export type { ExampleType };

You may ask "why is ExampleType a type alias?" Well, when you write

type MyTypeAlias = number;

You are explicitly creating the type alias MyTypeAlias which aliases number. And when you write

import type { YourTypeAlias } from './YourModule';

You are implicitly creating the type alias YourTypeAlias which aliases the YourTypeAlias export of YourModule.js.

answered Sep 22 '15 at 21:49

Gabe Levi

but how do you easily re-export all types in one line? – faceyspacey.com Feb 11 '17 at 02:49
2

According to this post, `export type {ExampleType}` is not a supported syntax. https://flowtype.org/blog/2015/02/18/Import-Types.html Is the information in the post out of date? Where is this syntax documented? It is not documented in https://flowtype.org/docs/modules.html either. – Gajus Mar 24 '17 at 11:37
FYI, I have filled a related issue with Babel https://github.com/babel/babel/issues/5538 regarding the `babel-plugin-transform-flow-comments`. – Gajus Mar 24 '17 at 11:51
For the record: There is an announcement of that syntax on the bottom of the page @Gajus linked to. I tried (Flow 0.46) and it works, that blog post is very old. – Mörre May 27 '17 at 13:29
1

NOTE: `export type * from './foo'` doesn't seem to work. – vaughan Mar 10 '18 at 20:18

locropulenton · Answer 2 · 2019-09-11T09:11:54.857

16

The following works nicely

export type { Type } from './types';

edited Sep 11 '19 at 09:11

answered May 25 '17 at 06:10

locropulenton

score 8 · Answer 3 · answered Feb 24 '18 at 02:28

The accepted answer is old and throwing warnings on my end. Given the amount of views, here's an updated answer that's compatible with flow 0.10+ .

MyTypes.js:

  export type UserID = number;
  export type User = {
    id: UserID,
    firstName: string,
    lastName: string
  };

User.js:

  import type {UserID, User} from "MyTypes";

  function getUserID(user: User): UserID {
    return user.id;
  }

score 1 · Answer 4 · answered May 26 '18 at 04:03

I just found a need one-liner to do this for ES6 default classes, building on @locropulenton's answer. Suppose you have

// @flow

export default class Restaurants {}

in a Restaurants.js file. To export it from an index.js file in the same directory, do this:

export type {default as Restaurants} from './Restaurants';

4 Answers4