Checking third-party code

Most real JavaScript programs depend on third-party libraries. To handle this, Flow supports the concept of a “Library Definition” (or “libdef” for short).

Library Definitions

Often you will want to use libraries and code that you didn’t write – such as npm packages. For these circumstances, Flow supports the concept of a “libdef” which allows you to describe the interface and types of the library separate from the library and without needing to add types to or change the library itself.

It’s always possible to write a libdef file yourself if you need to, but for public libraries we recommend using flow-typed. flow-typed is a community-run project that organizes and shares existing, high-quality libdefs for popular libraries.

If you can’t find the libdef you need there, consider writing one and then contributing it back so that the next person who needs it can benefit!

Note that if a 3rd party library is present in your project that isn’t itself typed, Flow will treat it just like any other untyped JS file in your project: It and all imports from it will be marked as any by default. This is the only place in Flow where any is ever injected in to your program by Flow.

Using flow-typed

Using the flow-typed project for npm projects is pretty straightforward. First, make sure you have the flow-typed CLI installed:

$> npm install -g flow-typed

Then, from within your project directory, run flow-typed install after running npm install:

$> npm install
$> flow-typed install

The flow-typed install command will do the following:

  1. Read your package.json to identify the dependencies of your project
  2. For each dependency, look in the flow-typed repo for a compatible libdef.
  3. For each dependency:
    • If a compatible libdef is found, download it into the <PROJECT_ROOT>/flow-typed/ directory.
    • If no compatible libdef is found and the library does not come with types, generate a stub libdef so that you can go fill in the types yourself. Don’t forget to contribute your work back up to flow-typed when you’re done :)

Since libdefs can be improved over time, we recommend that you commit all libdefs into version control. This ensures that all developers working in your project’s repository have a consistent and predictable set of types. It is good to periodically run flow-typed update to check and see if there have been any improvements or bug fixes made to the libdefs you’ve installed for your project.

Writing custom library definition files

Library definition files (“libdefs”) define the type interface to a library separately from the actual code of the library. You never need to change library code to use a library definition, but your code will be typechecked against the types declared in the libdef.

For libdefs of public libraries, consider using and contributing to flow-typed. If you have private libraries or just want to write your own libdefs, here is the workflow for doing so:

  • Do not change the library files or add @flow to them
  • Add one or more libdefs for your libraries in to the “<PROJECT_ROOT>/flow-typed/” directory of your project. Flow recognizes this directory as special by default and interprets it as a directory that contains libdef files.

If you need to customize the directory where libdefs are stored in your project, you can do so by adding a [libs] configuration to your .flowconfig file:


Example: Writing a custom libdef

To illustrate this workflow, we’ll start with the lodash library. Let’s say we have a simple file in our project using lodash:


// @flow

import _ from "lodash";

var pizzas = [
  { title: 'Margherita', vegetarian: true },
  { title: 'Pepperoni', vegetarian: false },
  { title: 'Four cheese', vegetarian: true },
  { title: 'Hawaiian', vegetarian: false },

function vegetarianPizzas() {
  return _.ffind(pizzas, {vegetarian: true});

Running flow will produce no errors:

$> flow
Found 0 errors

If you look closely at our example, though, you’ll notice there is a bug! We’ve misspelled find as ffind on line 13.

The problem here is that Flow doesn’t know anything about the lodash library since it is untyped (i.e. its code lacks @flow comments). To fix this we need a libdef file for lodash – so let’s write one ourselves:


declare module "lodash" {
  declare module.exports: {
    find<T>(list: Array<T>, properties: Object): T;

Here we’ve written a file that contains a declare module statement. declare module inside a libdef tells Flow about the interface defined by a module of the given name.

Within the body of this statement, we have another statement: declare module.exports: .... This is how we explain to Flow what the type of the module.exports object inside this module should be. In this case, we say that it is an object type with one find method.

Everything after the : in “declare module.exports: ...” is just a normal type annotation, so you can read more about type annotations here.

Now we can run flow again and see that Flow gives us an error as we’d expect:

$> flow
 13:   return _.ffind(pizzas, {vegetarian: true});
                ^^^^^ property `ffind`. Property not found in
 13:   return _.ffind(pizzas, {vegetarian: true});
              ^ object type

Found 1 error

If we fix the typo in our code and run Flow again, we’ll see that the Flow error goes away:

$> flow
Found 0 errors

For more information about declare statements in libdefs, see the reference guide on declarations.

© 2013–present Facebook Inc.
Licensed under the BSD License.