Higher-order Components

Learn how to use Flow to type React higher-order components

A popular pattern in React is the higher-order component pattern, so it’s important that we can provide effective types for higher-order components in Flow. If you don’t already know what a higher-order component is then make sure to read the React documentation on higher-order components before continuing.

Table of contents:

HOCs as of 0.89.0

In 0.89.0, we introduced React.AbstractComponent, which makes writing HOCs and library definitions very expressive.

Let’s take a look at how you can type some example HOCs.

The Trivial HOC

Let’s start with the simplest HOC:

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//@flow
import * as React from 'react';

function trivialHOC<Config: {}>(
  Component: React.AbstractComponent<Config>
): React.AbstractComponent<Config> {
  return Component;
}

This is a basic template for what your HOCs might look like. At runtime, this HOC doesn’t do anything at all. Let’s take a look at some more complex examples.

Injecting Props

A common use case for higher-order components is to inject a prop. The HOC automatically sets a prop and returns a component which no longer requires that prop. For example, consider a navigation prop, or in the case of react-redux a store prop. How would one type this?

To remove a prop from the config, we can use $Diff. Let’s make some small adjustments to our trivialHoc:

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//@flow
import * as React from 'react';

function injectProp<Config: {}>(
  Component: React.AbstractComponent<Config>
): React.AbstractComponent<$Diff<Config, {foo: number | void}>> {
  return function WrapperComponent(
    props: $Diff<Config, {foo: number | void}>,
  ) {
    return <Component {...props} foo={42} />;
  };
}

class MyComponent extends React.Component<{
  a: number,
  b: number,
  foo: number,
}> {}

const MyEnhancedComponent = injectProp(MyComponent);

// We don't need to pass in `foo` even though `MyComponent` requires it.
<MyEnhancedComponent a={1} b={2} />;

Preserving the Instance Type of a Component

Recall that the instance type of a function component is void. Our example above wraps a component in a function, so the returned component has the instance type void.

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//@flow
import * as React from 'react';

function injectProp<Config: {}>(
  Component: React.AbstractComponent<Config>
): React.AbstractComponent<$Diff<Config, {foo: number | void}>> {
  return function WrapperComponent(
    props: $Diff<Config, {foo: number | void}>,
  ) {
    return <Component {...props} foo={42} />;
  };
}

class MyComponent extends React.Component<{
  a: number,
  b: number,
  foo: number,
}> {}

const MyEnhancedComponent = injectProp(MyComponent);

// If we create a ref object for the component, it will never be assigned
// an instance of MyComponent!
const ref = React.createRef<MyComponent>();

// Error, mixed is incompatible with MyComponent.
<MyEnhancedComponent ref={ref} a={1} b={2} />;
Cannot create `MyEnhancedComponent` element because in property `ref`: Either mixed [1] is incompatible with `MyComponent` [2] in property `current`. Or a call signature declaring the expected parameter / return type is missing in object type [3] but exists in function type [4].

We get this error message because React.AbstractComponent<Config> doesn’t set the Instance type parameter, so it is automatically set to mixed. If we wanted to preserve the instance type of the component, we can use React.forwardRef:

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//@flow
import * as React from 'react';

function injectAndPreserveInstance<Config: {}, Instance>(
  Component: React.AbstractComponent<Config, Instance>,
): React.AbstractComponent<Config, Instance> {
  return React.forwardRef<Config, Instance>((props, ref) =>
      <Component ref={ref} {...props} />
  );
}

class MyComponent extends React.Component<{}> {}

const MyEnhancedComponent = injectAndPreserveInstance(MyComponent);

const ref = React.createRef<MyComponent>();

// All good! The ref is forwarded.
<MyEnhancedComponent ref={ref} a={1} b={2} />;

Exporting Wrapped Components

If you try to export a wrapped component, chances are that you’ll run into a missing annotation error:

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//@flow
import * as React from 'react';

function trivialHOC<Config: {}>(
  Component: React.AbstractComponent<Config>,
): React.AbstractComponent<Config> {
  return Component;
}

type DefaultProps = {| foo: number |};
type Props = {...DefaultProps, bar: number};

class MyComponent extends React.Component<Props> {
  static defaultProps: DefaultProps = {foo: 3};
}

// Error, missing annotation for Config.
const MyEnhancedComponent = trivialHOC(MyComponent);

module.exports = MyEnhancedComponent;
Missing type annotation for `Config`. `Config` is a type parameter declared in function [1] and was implicitly instantiated at call of `trivialHOC` [2].

If your component has no defaultProps, you can use Props as a type argument for Config.

If your component does have defaultProps, you don’t want to just add Props as a type argument to trivialHOC because that will get rid of the defaultProps information that flow has about your component.

This is where React.Config<Props, DefaultProps> comes in handy! We can use the type for Props and DefaultProps to calculate the Config type for our component.

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//@flow
import * as React from 'react';

function trivialHOC<Config: {}>(
  Component: React.AbstractComponent<Config>,
): React.AbstractComponent<Config> {
  return Component;
}

type DefaultProps = {| foo: number |};
type Props = {...DefaultProps, bar: number};

class MyComponent extends React.Component<Props> {
  static defaultProps: DefaultProps = {foo: 3};
}

const MyEnhancedComponent = trivialHOC<React.Config<Props, DefaultProps>>(MyComponent);

// Ok!
module.exports = MyEnhancedComponent;

HOCs before 0.89.0

To learn how to type higher-order components we will look to Recompose for examples of higher-order components. Recompose is a popular React library that provides many higher-order components. Let’s see how you would type the mapProps() higher-order component from Recompose.

mapProps() takes a function that will transform the input props into some output props. You can use mapProps() like this:

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function MyComponent({ bar }: { bar: number }) {
  return <div>{bar}</div>;
}

const MyEnhancedComponent = mapProps(
  ({ foo }) => ({ bar: foo + 1 }),
)(MyComponent);

<MyEnhancedComponent foo={1} />; // This will render the number 2.

For the type of MyComponent and the type of MyEnhancedComponent we will use React.ComponentType<Props>. React.ComponentType<Props> is a union of stateless functional components and class components where Props is the defined type for the component’s props.

We want mapProps() to return a function that will take a React component as its first and only argument and return a React component.

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import * as React from 'react';

function mapProps(): (React.ComponentType<any>) => React.ComponentType<any> {
  return Component => {
    // implementation...
  };
}

Remember: We are returning a function type here and we are taking no arguments. We will add the argument (which is also a function) in a later step.

To start we used any for our React.ComponentType<Props>s’ Props types! So next we will use a generic function type to provide better types than any.

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import * as React from 'react';

function mapProps<PropsInput: {}, PropsOutput: {}>(
  // TODO
): (React.ComponentType<PropsOutput>) => React.ComponentType<PropsInput> {
  return Component => {
    // implementation...
  };
}

Note that PropsInput and PropsOutput have bound of {}. (As expressed in PropsInput: {} and PropsOutput: {}.) This means that PropsInput and PropsOutput must be object types so we may treat them as such in the implementation of mapProps(). If you do not add the : {} then you would not be able to spread PropsInput or PropsOutput!

We have one last thing to do. Add a type for the mapper function which will take PropsInput and return PropsOutput for mapProps().

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import * as React from 'react';

function mapProps<PropsInput: {}, PropsOutput: {}>(
  mapperFn: (PropsInput) => PropsOutput,
): (React.ComponentType<PropsOutput>) => React.ComponentType<PropsInput> {
  return Component => {
    // implementation...
  };
}

Now you can use mapProps() with confidence that Flow is ensuring your types are correct.

Note: While when you use mapProps() in the following:

const MyEnhancedComponent = mapProps(
  ({ foo }) => ({ bar: foo + 1 }),
)(MyComponent);

Flow will not require you to add type annotations, but it is a smart idea to add annotations anyway. By adding type annotations you will get better error messages when something is broken. An annotated version of a mapProps() usage would look like:

const MyEnhancedComponent = mapProps(
  (props: PropsA): PropsB => /* ... */,
)(MyComponent);

Where PropsA and PropsB are your type annotations.

Injecting Props With a Higher-order Component

A common use case for higher-order components is to inject a prop. Like a navigation prop, or in the case of react-redux a store prop. How would one type this? Let us start with a higher-order component that does not add any new props:

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import * as React from 'react';

function injectProp<Props: {}>(
  Component: React.ComponentType<Props>,
): React.ComponentType<Props> {
  // implementation...
}

This generic function type will take a React component and return a React component with the exact same type for props. To remove a prop from the returned component we will use $Diff.

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import * as React from 'react';

function injectProp<Props: {}>(
  Component: React.ComponentType<Props>,
): React.ComponentType<$Diff<Props, { foo: number | void }>> {
  // implementation...
}

Let’s look at the type for our output component. In other words the type for MyOutputComponent in const MyOutputComponent = injectProp(MyInputComponent).

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React.ComponentType<$Diff<Props, { foo: number | void }>

The type of props for this component is:

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$Diff<Props, { foo: number | void }>

This uses $Diff to say that the type for props is everything in Props (which is the props type for our output component) except for foo which has a type of number.

Note: If foo does not exist in Props you will get an error! $Diff<{}, { foo: number }> will be an error. To work around this use a union with void, see: $Diff<{}, { foo: number | void }>. An optional prop will not completely remove foo. $Diff<{ foo: number }, { foo?: number }> is { foo?: number } instead of {}.

With this we can now use injectProp() to inject foo.

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import * as React from 'react';

function injectProp<Props: {}>(
  Component: React.ComponentType<Props>,
): React.ComponentType<$Diff<Props, { foo: number | void }>> {
  return function WrapperComponent(props: Props) {
    return <Component {...props} foo={42} />;
  };
}

class MyComponent extends React.Component<{
  a: number,
  b: number,
  foo: number,
}> {}

const MyEnhancedComponent = injectProp(MyComponent);

// We don't need to pass in `foo` even though `MyComponent` requires it.
<MyEnhancedComponent a={1} b={2} />;

Note: Remember that the generic type, Props, needs the bound {}. As in Props: {}. Otherwise you would not be able to spread Props in <Component {...props} foo={42} />.

Supporting defaultProps With React.ElementConfig<>

The higher-order-components we’ve typed so far will all make defaultProps required. To preserve the optionality of defaultProps you can use React.ElementConfig<typeof Component>. Your enhancer function will need a generic type for your component. Like this:

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function myHOC<Props, Component: React.ComponentType<Props>>(
  WrappedComponent: Component
): React.ComponentType<React.ElementConfig<Component>> {
  return props => <WrappedComponent {...props} />;
}

Notice here how we used React.ComponentType<React.ElementConfig<Component>> as the output component type instead of React.ComponentType<Props> as we’ve seen in previous examples.

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