Importing And Using Type Definitions

Tane Morgan edited this page Sep 29, 2017 · 4 revisions

This guide assumes you've followed the installation steps in the README, and that you've installed the the definitions for your project's dependencies.

Existing Definitions

NOTE: Using the /flow-typed directory for libdefs is a convention that enables Flow to JustWork™ out of the box and encourages consistency across projects that use Flow, but it is also possible to explicitly configure Flow to look elsewhere for libdefs using the [libs] section of your .flowconfig. [1]

Once you've installed the definitions (via flow-typed install...), you can start using them right away. For instance, if you're using underscore methods, simply import the underscore library:

import _ from 'underscore';

let minimum: number = _.min([1, 3, 4, 2]);

If the definition includes type declarations, you can import them with import type:

import { reduxForm } from 'redux-form';
import type { FormProps } from 'redux-form';  

Manual/Custom Definitions

Note: The flow docs has a great a page on Creating Library Definitions.

If you're unable to find a definition using flow-typed, you can simply create it yourself. Start by creating a folder called custom-definitions at the root of your project, and adding it to the [libs] section of the .flowconfig file.


Define your custom modules by adding them to the folder; in our case, a definitions for awesome-library would exist in ./custom-definitions/awesome-library.js.

declare module 'awesome-library' {
  declare type AwesomeType = {
    name: string,
    reason: string,
    is_awesome: boolean
  declare function makeAwesome(name: string): AwesomeType;

In the file where you wish to use awesome-library, import both it and the type definition.

import Awesome from 'awesome-library';
import type { AwesomeType } from 'awesome-library';

And you're done! You can now use the type checked version of makeAwesome, and type check the return value.

let awesomeThing: AwesomeType = Awesome.makeAwesome('thing');

This wiki page was created to clarify issue #438.

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