Higher-order Components - Flow

:::danger Higher-order components are discouraged in modern React code and will not be updated for Component Syntax. Consider using a hook to accomplish your task instead. :::

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.

You can make use of the Component Types to annotate your higher order components.

The Trivial HOC {#toc-the-trivial-hoc}

Let's start with the simplest HOC:

import * as React from 'react';

function trivialHOC<Config extends {...}>(
  Component: component(...Config),
): component(...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 {#toc-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. How would one type this?

To remove a prop from the config, we can take a component that includes the prop and return a component that does not. It's best to construct these types using object type spread.

import * as React from 'react';

type InjectedProps = {foo: number}

function injectProp<Config extends {...}>(
  Component: component(...{...Config, ...InjectedProps, ...})
): component(...Config) {
  return function WrapperComponent(
    props: Config,
  ) {
    return <Component {...props} foo={42} />;
  };
}

function MyComponent(props: {
  a: number,
  b: number,
  ...InjectedProps,
  ...
}): React.Node {}

const MyEnhancedComponent = injectProp(MyComponent);

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

// We still require `a` and `b`:
<MyEnhancedComponent a={1} />; // ERROR

Preserving the Instance Type of a Component {#toc-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.

import * as React from 'react';

type InjectedProps = {foo: number}

function injectProp<Config>(
  Component: component(...{...Config, ...InjectedProps})
): component(...Config) {
  return function WrapperComponent(
    props: Config,
  ) {
    return <Component {...props} foo={42} />;
  };
}

// A class component in this example
class MyComponent extends React.Component<{
  a: number,
  b: number,
  ...InjectedProps,
}> {}

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} />;

We get this error message because component type doesn't declare the ref prop, so it is treated as React.RefSetter<void>. If we wanted to preserve the instance type of the component, we can use React.forwardRef:

import * as React from 'react';

type InjectedProps = {foo: number}

function injectAndPreserveInstance<Config extends {...}, Instance>(
  Component: component(ref?: React.RefSetter<Instance>, ...{...Config, ...InjectedProps})
): component(ref?: React.RefSetter<Instance>, ...Config) {
  return React.forwardRef<Config, Instance>((props, ref) =>
      <Component ref={ref} foo={3} {...props} />
  );
}

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

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 {#toc-exporting-wrapped-components}

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

import * as React from 'react';

function trivialHOC<Config extends {...}>(
  Component: component(...Config),
): component(...Config) {
  return Component;
}

type Props = Readonly<{bar: number, foo?: number}>;

function MyComponent({bar, foo = 3}: Props): React.Node {}

export const MyEnhancedComponent = trivialHOC(MyComponent); // ERROR

You can add an annotation to your exported component using component types:

import * as React from 'react';

function trivialHOC<Config extends {...}>(
  Component: component(...Config),
): component(...Config) {
  return Component;
}

type Props = Readonly<{bar: number, foo?: number}>;

function MyComponent({bar, foo = 3}: Props): React.Node {}

export const MyEnhancedComponent: component(...Props) = trivialHOC(MyComponent); // OK