Flow Cookbook: Flow & React

This recipe is part of the Flow Cookbook series.

Flow and React are both Facebook projects - so as you might imagine, they work quite well together. React components can take type parameters to specify types for props and state. Type-checking works well with both functional and class components.

Flow type annotations provide an alternative to propTypes runtime checks. Flow’s static checking has some advantages:

  • Problems are reported immediately - it is not necessary to run tests that evaluate every component to identify props mismatches.
  • Flow types can be more precise and concise than propTypes.
  • Flow can also check state as well as props.
  • In addition to checking that a component gets the correct props, Flow checks that props and states are used correctly within the component’s render method, and in other component methods.

Flow changed the way that it handles React types in version 0.53.0. This recipe assumes that you are using Flow v0.53.0 or later.

How it works

With Flow you specify the types of props and state using type parameters on the React.Component class. The form for type parameters is:

class MyComponent extends React.Component<Props, State> {
  /* ... */

Where Props, and State can be whatever object types you want. If your component has the props title, createdAt, and authorId then a definition for Props could look like this:

type Props = {
  title: string,
  createdAt: Date,
  authorId: number,

If your component is stateless then you can omit the State parameter, in which case the state type defaults to void.

Props should list types for all props - even props that are optional or that have default values. If you add a defaultProps value to your component then Flow will automatically infer that fields from defaultProps are not required when your component is called. So you should not use a ? in the corresponding field in your Props type. Leaving out the ? lets you avoid unnecessary undefined checks in your component’s methods.

type Props = {
  updatedAt?: Date,      // optional prop, no default value
  commentsEnabled: bool, // optional, but has a default value in `defaultProps`

class MyComponent extends React.Component<Props> {
  static defaultProps = { commentsEnabled: true }

  /* ... */

To declare a type for props in a stateless functional component use a type annotation like this:

function MyComponent(props: Props) {
  /* ... */

Once again, Props can be any object type that you want, and should include types for all props, even those that have default values. The return type for a functional component is React.Element<*> - but Flow can infer that so don’t bother with a return type annotation.

Type definitions for React are built into Flow. The Flow documentation includes a type reference for React types. It is useful to look at those definitions to see exactly what Flow expects. You can also go straight to the source: a lot of what I know about using Flow came from examining Flow’s type definition file for React and other files in the same directory.

The Flow documentation also includes its own guide on using Flow with React.

A Hacker News client

Let’s build on the code from the Unpacking JSON API data recipe, and build a simple Hacker News client. The client will fetch lists of stories from the Hacker News API to display. When the user selects a story, the client will fetch and display comments.

Let’s start with a component that displays a Hacker News story as a single line, and accepts a callback to do something if the user selects that story. First we define a type for the props that this component will accept.

import { type Story } from 'flow-cookbook-hacker-news'

type StoryListItemProps = {
  story: Story,
  onSelect: () => void,

We import the Story type from 'flow-cookbook-hacker-news', which is the example code from the Unpacking JSON API data recipe. That Story type is a ready-made type that describes everything we will want to display in the new client. Take a look at the source file to see what Story looks like. (The import { type T } bit is Flow syntax - it is not part of Javascript.)

The StoryListItemProps type lists the props that will be given to our component. The props are a story, and a callback called onSelect. The type indicates that onSelect must be a function that takes zero arguments, and that returns undefined (a.k.a. void). It is not necessary to require that the return type be undefined - you could use mixed instead if you do not want to put any constraint on the callback’s return type.

My opinion is that using void is a clear indication to the caller that the component will not do anything with a return value. On the other hand you might get an irritating type error if you provide a single-expression arrow function that implicitly returns a value.

And here is the StoryListItem component itself:

function StoryListItem(props: StoryListItemProps) {
  const { by, title } = props.story
  return <p>
    <a href="#" onClick={event => selectStory(props, event)}>
    </a> posted by {by}

function selectStory(props: StoryListItemProps, event: Event) {

The type annotation, props: StoryListItemProps, does two things for us: when StoryListItem is rendered, Flow will check that it is given the required props of the appropriate types; and Flow will also check uses of props in the body of the StoryListItem function for consistency with StoryListItemProps. Checking props from both directions ensures that a component is in alignment with its callers.

In general if a component does not have internal state, and does not use life cycle callbacks such as componentDidMount, then I prefer to use a functional component, like StoryListItem. A functional component cannot have methods that refer to this.props; so I made the selectStory event handler a top-level function that accepts a reference to the component’s props as an argument. Reusing the StoryListItemProps type in the signatures for the component and for the event handler means that we can easily keep track of available props in both functions.

selectStory also takes an event argument. The Event type is built into Flow, and Flow knows about the preventDefault method. Flow’s version of the Event type is defined in Flow’s DOM type definitions file.

To populate instances of the StoryListItem component, we will need to make API requests and update some state. Here is a type for that state, and the parameters for the top-level component, which will manage that state:

type AppProps = {
  numStories: number,

type AppState = {
  selectedStory?: ?Story,
  stories?: Story[],
  error?: Error,

class App extends React.Component<AppProps, AppState> {
  constructor(props: AppProps) {
    this.state = {}

  /* ... */

The definition of AppState shows that the state will hold an array of stories, which will not be defined while stories are loading; a selectedStory, which will be defined when the user is viewing comments on a story; and an error, in case something goes wrong while loading stories.

Because every property in AppState is optional, state can be initialized as an empty object. (The question mark at the end of a property name indicates that property might not be set.) selectedStory is optional, and its type is nullable, so there are two question marks on that line. That is because selectedStory is initially not set, and after a user views a story and then backs out to the story list selectedStory will be set to null.

Since App has a constructor, we specify the props type both in the second class type parameter, and again in the type of the props argument to constructor. That is because a subclass can define a constructor with a signature that differs from the parent class’ constructor. It would not make much sense to do that in a React Component; but Flow must be able to check all sorts of classes, so it does not make assumptions about types of constructor arguments.

App will fetch stories when it is mounted. So we extend its definition with a componentDidMount callback.

import { fetchTopStories } from 'flow-cookbook-hacker-news'

class App extends React.Component<AppProps, AppState> {
  /* ... same as before */

  componentDidMount() {
    fetchTopStories(this.props.numStories /* number of stories to fetch */)
      .then(stories => {
        // On success, update component state with an array of stories
        this.setState({ stories })
      .catch(error => {
        // On error, update state to capture the error for display
        this.setState({ error })

  /* ... */

fetchTopStories takes an argument that specifies the number of stories to fetch, and it returns a promise.

Once stories are loaded we can display them via a render method. But the component will initially display while the API request is loading; so we will have to check whether stories have loaded, and display a loading indicator if they are not ready.

class App extends React.Component<AppProps, AppState> {
  /* ... */

  render() {
    const { error, selectedStory, stories } = this.state

    let content
    if (error) {
      // an error occurred fetching stories
      content = <p className="error">{error.message}</p>
    else if (selectedStory) {
      // the user is looking at comments on a story
      content = <StoryView
        onNavigateBack={() => this.deselectStory()}
    else if (stories) {
      // stories are loaded, so display a line item for each story
      content = stories.map(story => (
          onSelect={() => this.selectStory(story)}
    else {
      // no error and no stories means that stories are still loading
      content = <p className="loading">loading...</p>

    return (
      <div className="App">
        <div className="App-header">
          <h1>Flow Cookbook: React Example</h1>

  /* ... */

The import piece of the JSX that is finally returned from render is content, which is assigned a different value depending on whether an error occurred while fetching stories, a specific story has been selected, or there is an array of stories available, and none has been selected. If none of those cases applies, then it means that stories are still loading.

When displaying a list of stories, content is populated by a list of instances of StoryListItem, which we defined earlier. There is nothing special about rendering a type-checked component - you pass props the same way as with any component.

Flow can track changes to the type of a variable over a series of statements. Flow infers that when content is first defined its type is void; and it infers that no matter which code path gets executed, by the time we get to the return statement the type has changed to either React.Element<*> or React.Element<*>[] - either of which is compatible with Flow’s expectations for JSX content.

The event handling methods selectStory and deselectStory make state changes to track whether the user is looking at a specific story:

class App extends React.Component<AppProps, AppState> {
  /* ... */

  selectStory(story: Story) {
    this.setState({ selectedStory: story })

  deselectStory() {
    this.setState({ selectedStory: null })

Those state changes affect the output from render to switch between displaying a list of story titles, or a detail view for a single story.

At this point we have made references to App’s state in componentDidMount, render, selectStory, and deselectStory. Flow checks all of those uses against the definition of AppState. For example, if we made a mistaken assumption that selectedStory holds an ID, as opposed to a value of type Story, and tried to write something like this.setState({ selectedStory: story.id }) Flow would report an error, and point out the mismatch.

I have not given the implementation of StoryView, which is responsible for displaying comments on a story. It happens that StoryView is quite similar to App - except that StoryView loads comments where App loads stories. The interested reader can see the full details of StoryView in the accompanying code.

Complete working code is available at https://github.com/hallettj/flow-cookbook-react. Your next assignment is to clone that code and to add some features. Some ideas to try are to add links to the original article for each story, or to display profile pages for posters and commenters.

The example code here uses React’s own state features to manage app state. That helps to keep this recipe self-contained. But in my opinion the best practice is to keep state in a third-party state-management framework such as Redux. For details on using Flow with Redux and react-redux take a look at the next recipe, Flow & Redux.


2017-01-07 — The hacker news client library is now on npm - updated references accordingly
2017-11-22 — Updates for Flow v0.53.0