Section 1: React HOC docs in TypeScript

In this first section we refer closely to the React docs on HOCs and offer direct TypeScript parallels.

Docs Example: Use HOCs For Cross-Cutting Concerns

Misc variables referenced in the example below
/** dummy child components that take anything */
const Comment = (_: any) => null;
const TextBlock = Comment;
/** dummy Data */
type CommentType = { text: string; id: number };
const comments: CommentType[] = [
{
text: "comment1",
id: 1,
},
{
text: "comment2",
id: 2,
},
];
const blog = "blogpost";
/** mock data source */
const DataSource = {
addChangeListener(e: Function) {
// do something
},
removeChangeListener(e: Function) {
// do something
},
getComments() {
return comments;
},
getBlogPost(id: number) {
return blog;
},
};
/** type aliases just to deduplicate */
type DataType = typeof DataSource;
// type TODO_ANY = any;
/** utility types we use */
type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
// type Optionalize<T extends K, K> = Omit<T, keyof K>;
/** Rewritten Components from the React docs that just uses injected data prop */
function CommentList({ data }: WithDataProps<typeof comments>) {
return (
<div>
{data.map((comment: CommentType) => (
<Comment comment={comment} key={comment.id} />
))}
</div>
);
}
interface BlogPostProps extends WithDataProps<string> {
id: number;
// children: ReactNode;
}
function BlogPost({ data, id }: BlogPostProps) {
return (
<div key={id}>
<TextBlock text={data} />;
</div>
);
}

View in TypeScript Playground

Example HOC from React Docs translated to TypeScript

// these are the props to be injected by the HOC
interface WithDataProps<T> {
data: T; // data is generic
}
// T is the type of data
// P is the props of the wrapped component that is inferred
// C is the actual interface of the wrapped component (used to grab defaultProps from it)
export function withSubscription<T, P extends WithDataProps<T>, C>(
// this type allows us to infer P, but grab the type of WrappedComponent separately without it interfering with the inference of P
WrappedComponent: React.JSXElementConstructor<P> & C,
// selectData is a functor for T
// props is Readonly because it's readonly inside of the class
selectData: (
dataSource: typeof DataSource,
props: Readonly<JSX.LibraryManagedAttributes<C, Omit<P, "data">>>
) => T
) {
// the magic is here: JSX.LibraryManagedAttributes will take the type of WrapedComponent and resolve its default props
// against the props of WithData, which is just the original P type with 'data' removed from its requirements
type Props = JSX.LibraryManagedAttributes<C, Omit<P, "data">>;
type State = {
data: T;
};
return class WithData extends React.Component<Props, State> {
constructor(props: Props) {
super(props);
this.handleChange = this.handleChange.bind(this);
this.state = {
data: selectData(DataSource, props),
};
}
componentDidMount = () => DataSource.addChangeListener(this.handleChange);
componentWillUnmount = () =>
DataSource.removeChangeListener(this.handleChange);
handleChange = () =>
this.setState({
data: selectData(DataSource, this.props),
});
render() {
// the typing for spreading this.props is... very complex. best way right now is to just type it as any
// data will still be typechecked
return (
<WrappedComponent data={this.state.data} {...(this.props as any)} />
);
}
};
// return WithData;
}
/** HOC usage with Components */
export const CommentListWithSubscription = withSubscription(
CommentList,
(DataSource: DataType) => DataSource.getComments()
);
export const BlogPostWithSubscription = withSubscription(
BlogPost,
(DataSource: DataType, props: Omit<BlogPostProps, "data">) =>
DataSource.getBlogPost(props.id)
);

Docs Example: Don’t Mutate the Original Component. Use Composition.

This is pretty straightforward - make sure to assert the passed props as T due to the TS 3.2 bug.

function logProps<T>(WrappedComponent: React.ComponentType<T>) {
return class extends React.Component {
componentWillReceiveProps(
nextProps: React.ComponentProps<typeof WrappedComponent>
) {
console.log("Current props: ", this.props);
console.log("Next props: ", nextProps);
}
render() {
// Wraps the input component in a container, without mutating it. Good!
return <WrappedComponent {...(this.props as T)} />;
}
};
}

Docs Example: Pass Unrelated Props Through to the Wrapped Component

No TypeScript specific advice needed here.

Docs Example: Maximizing Composability

HOCs can take the form of Functions that return Higher Order Components that return Components.

connect from react-redux has a number of overloads you can take inspiration from in the source.

Here we build our own mini connect to understand HOCs:

Misc variables referenced in the example below
/** utility types we use */
type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>;
/** dummy Data */
type CommentType = { text: string; id: number };
const comments: CommentType[] = [
{
text: "comment1",
id: 1,
},
{
text: "comment2",
id: 2,
},
];
/** dummy child components that take anything */
const Comment = (_: any) => null;
/** Rewritten Components from the React docs that just uses injected data prop */
function CommentList({ data }: WithSubscriptionProps<typeof comments>) {
return (
<div>
{data.map((comment: CommentType) => (
<Comment comment={comment} key={comment.id} />
))}
</div>
);
}
const commentSelector = (_: any, ownProps: any) => ({
id: ownProps.id,
});
const commentActions = () => ({
addComment: (str: string) =>
comments.push({ text: str, id: comments.length }),
});
const ConnectedComment = connect(commentSelector, commentActions)(CommentList);
// these are the props to be injected by the HOC
interface WithSubscriptionProps<T> {
data: T;
}
function connect(mapStateToProps: Function, mapDispatchToProps: Function) {
return function <T, P extends WithSubscriptionProps<T>, C>(
WrappedComponent: React.ComponentType<T>
) {
type Props = JSX.LibraryManagedAttributes<C, Omit<P, "data">>;
// Creating the inner component. The calculated Props type here is the where the magic happens.
return class ComponentWithTheme extends React.Component<Props> {
public render() {
// Fetch the props you want inject. This could be done with context instead.
const mappedStateProps = mapStateToProps(this.state, this.props);
const mappedDispatchProps = mapDispatchToProps(this.state, this.props);
// this.props comes afterwards so the can override the default ones.
return (
<WrappedComponent
{...this.props}
{...mappedStateProps}
{...mappedDispatchProps}
/>
);
}
};
};
}

View in TypeScript Playground

Docs Example: Wrap the Display Name for Easy Debugging

This is pretty straightforward as well.

interface WithSubscriptionProps {
data: any;
}
function withSubscription<
T extends WithSubscriptionProps = WithSubscriptionProps
>(WrappedComponent: React.ComponentType<T>) {
class WithSubscription extends React.Component {
/* ... */
public static displayName = `WithSubscription(${getDisplayName(
WrappedComponent
)})`;
}
return WithSubscription;
}
function getDisplayName<T>(WrappedComponent: React.ComponentType<T>) {
return WrappedComponent.displayName || WrappedComponent.name || "Component";
}

Unwritten: Caveats section

  • Don’t Use HOCs Inside the render Method
  • Static Methods Must Be Copied Over
  • Refs Aren’t Passed Through