Higher-order Components

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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

Let's start with the simplest HOC:

1import * as React from 'react';2
3function trivialHOC<Config: {...}>(4  Component: component(...Config),5): component(...Config) {6  return Component;7}

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. 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.

1import * as React from 'react';2
3type InjectedProps = {foo: number}4
5function injectProp<Config>(6  Component: component(...{...$Exact<Config>, ...InjectedProps})7): component(...$Exact<Config>) {8  return function WrapperComponent(9    props: Config,10  ) {11    return <Component {...props} foo={42} />;12  };13}14
15function MyComponent(props: {16  a: number,17  b: number,18  ...InjectedProps,19}): React.Node {}20
21const MyEnhancedComponent = injectProp(MyComponent);22
23// We don't need to pass in `foo` even though `MyComponent` requires it:24<MyEnhancedComponent a={1} b={2} />; // OK25
26// We still require `a` and `b`:27<MyEnhancedComponent a={1} />; // ERROR
                    

27:2-27:20: Cannot create `MyEnhancedComponent` element because property `b` is missing in props [1] but exists in object type [2]. [prop-missing]

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.

1import * as React from 'react';2
3type InjectedProps = {foo: number}4
5function injectProp<Config>(6  Component: component(...{...$Exact<Config>, ...InjectedProps})7): component(...$Exact<Config>) {8  return function WrapperComponent(9    props: Config,10  ) {11    return <Component {...props} foo={42} />;12  };13}14
15// A class component in this example16class MyComponent extends React.Component<{17  a: number,18  b: number,19  ...InjectedProps,20}> {}21
22const MyEnhancedComponent = injectProp(MyComponent);
                                                  23
24// If we create a ref object for the component, it will never be assigned25// an instance of MyComponent!26const ref = React.createRef<MyComponent>();27
28// Error, mixed is incompatible with MyComponent.29<MyEnhancedComponent ref={ref} a={1} b={2} />;
                             

22:40-22:50: Cannot call `injectProp` with `MyComponent` bound to `Component` because `MyComponent` [1] is incompatible with instance of component [2]. [incompatible-call]
29:27-29:29: Cannot create `MyEnhancedComponent` element because in property `ref`: [incompatible-type] Either a call signature declaring the expected parameter / return type is missing in `React.RefObject` [1] but exists in function type [2]. Or `React.RefObject` [1] is incompatible with null [3].

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:

1import * as React from 'react';2
3type InjectedProps = {foo: number}4
5function injectAndPreserveInstance<Config: {...}, Instance>(6  Component: component(ref?: React.RefSetter<Instance>, ...{...$Exact<Config>, ...InjectedProps})7): component(ref?: React.RefSetter<Instance>, ...$Exact<Config>) {8  return React.forwardRef<$Exact<Config>, Instance>((props, ref) =>9      <Component ref={ref} foo={3} {...props} />10  );11}12
13class MyComponent extends React.Component<{14  a: number,15  b: number,16  ...InjectedProps,17}> {}18
19const MyEnhancedComponent = injectAndPreserveInstance(MyComponent);20
21const ref = React.createRef<MyComponent>();22
23// All good! The ref is forwarded.24<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:

1import * as React from 'react';2
3function trivialHOC<Config: {...}>(4  Component: component(...Config),5): component(...Config) {6  return Component;7}8
9type Props = $ReadOnly<{bar: number, foo?: number}>;10
11function MyComponent({bar, foo = 3}: Props): React.Node {}12
13export const MyEnhancedComponent = trivialHOC(MyComponent); // ERROR
                                                          

13:36-13:58: Cannot build a typed interface for this module. You should annotate the exports of this module with types. Cannot determine the type of this call expression. Please provide an annotation, e.g., by adding a type cast around this expression. [signature-verification-failure]

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

1import * as React from 'react';2
3function trivialHOC<Config: {...}>(4  Component: component(...Config),5): component(...Config) {6  return Component;7}8
9type Props = $ReadOnly<{bar: number, foo?: number}>;10
11function MyComponent({bar, foo = 3}: Props): React.Node {}12
13export const MyEnhancedComponent: component(...Props) = trivialHOC(MyComponent); // OK