Higher-Order Components with React & TypeScript

Higher-order components are a great way of reusing common functionality in your react app.

TypeScript is a great way of making sure your react app can grow without you losing your mind.

So how can we use the two together?

Higher-Order Components

A higher-order component (HoC) is a function that accepts a component as an argument and returns a new component that has been enhanced in some way.

const EnhancedComponent = myHigherOrderComponent(WrappedComponent);

The enhanced component can add behaviour, manage state or in any other way modify WrappedComponent.

Injecting Props

A common pattern is for a higher-order component to provide some props to the wrapped component, moving the management of those props out of WrappedComponent and allowing it to focus on other concerns.

For example, connect(...)(WrappedComponent) in react-redux provides props to both expose redux state and to dispatch actions. withRouter(WrappedComponent) in react-router similarly injects routing-related props into WrappedComponent from the current route.

Getting the prop types right when injecting props can be a bit fiddly. We want:

  1. WrappedComponent to have new props injected automatically
  2. Injected props not to be required on the created EnhancedComponent
  3. Any other props on WrappedComponent to be forwarded from EnhancedComponent
  4. TypeScript to infer all of the above with no manual specification for the user

For example, let’s say we have a HoC that will manage the isExpanded state of an expander component. It will provide the following props to WrappedComponent

interface ExpanderProps {
  isExpanded: boolean;
  toggleExpanded(): void;
}

Our wrapped component will need to accept these props but it might define some others as well. Let’s say it has a title that needs to be specified and an optional className:

interface WrappedComponentProps {
  isExpanded: boolean;
  toggleExpanded(): void;
  title: string;
  className?: string;
}

note that you could (and should!) define this as WrappedComponentProps extends ExpanderProps... and avoid duplicating the prop definitions. I’ve left this as it is here for clarity

In this case we want both title and className to be available on EnhancedComponent:

const EnhancedComponent = withExpander(WrappedComponent);

const usage = (
  <EnhancedComponent
    title="The title"
    className="class-name">
    ...
  </EnhancedComponent>
);

The Wrapped Component

At this stage we have already defined the provided props as an interface and we have an example of what the wrapped component might look like. Let’s flesh that out a bit (and give it a more appropriate name):

interface ExpanderComponentProps extends ExpanderProps {
  title: string;
}

class ExpanderComponent extends PureComponent<ExpanderComponentProps> {
  render() {
    const { isExpanded, toggleExpanded, title, children } = this.props;

    return (
      <>
        <button onClick={toggleExpanded}>{title}</button>
        {isExpanded && children}
      </>
    );
  }
}

Fairly simple: render a button containing title that toggles the expanded state. If isExpanded=true then also render the children.

Create the Enhancer

Now we want to create the enhancer function that we can use to inject isExpanded and toggleExpanded. Our first requirement is that it accepts an argument of WrappedComponent where WrappedComponent accepts the props we want to inject. That function signature might look something like this:

function withExpander(WrappedComponent: ComponentType<ExpanderProps>) {
  //...
}

ComponentType is a react type that allows you to pass in either a component class or a stateless functional component that has props of T

The next requirement is that it returns a component that doesn’t require the ExpanderProps to be specified, which means that we need to provide them from the enhanced component. In this case we’re going to get those values from the state on the component but that’s not a requirement – they can come from anywhere.

Once we have the values we need we can pass them to WrappedComponent in the render method:

function withExpander(WrappedComponent: ComponentType<ExpanderProps>) {
  return class WithExpander extends PureComponent<{}, { isExpanded: boolean }> {
    render() {
      return <WrappedComponent
        {...this.state}
        toggleExpanded={this.toggleExpanded}
        />
    }

    private toggleExpanded = () => this.setState(state => ({ isExpanded: !state.isExpanded }));
  }
}

All right, nice and easy! Except… build failure!

const EnhancedComponent = withExpander(ExpanderComponent);
// Error!

Our ExpanderComponent requires a title prop; when we use it (as WrappedComponent) in withExpander we don’t specify the title!

We could make withExpander aware of the title prop but that’s not very useful for the future: there will likely be other props that need to be passed through and we don’t want to keep adding them to our HoC.

This brings us on to Requirement 3 above: any other props from the wrapped component should be forwarded from the enhanced component. In this case, we really want to have the title prop be exposed on EnhancedComponent.

The first step in getting to that point is to allow the WrappedComponent parameter of our HoC to accept props that aren’t on ExpanderProps. In fact, we need our wrapped component to accept props of both ExpanderProps and whatever else it wants.

We can introduce a generic type parameter to the HoC function to represent that combined props object:

function withExpander<TWrappedComponentProps extends ExpanderProps>(
  WrappedComponent: ComponentType<TWrappedComponentProps> ) {
  //...
}

That solves our build failure: withExpander will now accept any component as long as it has the two isExpanded and toggleExpanded props.

Sadly we’re not quite done yet though; try to use the enhanced component and you will see our next problem:

const EnhancedComponent = withExpander(ExpanderComponent);

const usage = <EnhancedComponent title="Some title" />;
// Error!

Another build failure, this time telling us that title does not exist on the enhanced component. We’ve managed to write our HoC so that it allows extra props but all it does is ignore them!

What we really want here is to create a new Props type that we can assign to our created component. TypeScript 2.8 includes 2 built-in types that are going to help us out:

  • Pick<T, U> that takes the props from T specified in U. e.g. Pick<ExpanderProps, ‘isExpanded’> creates a type with only the isExpanded property
  • Exclude<T, U> that takes everything from T except U. e.g. Exclude<‘one’ | ‘two’ | ‘three’, ‘two’>* creates a type 'one' | 'three'

By combining these with keyof we can build up the “all props that are on the inner component but excluding the ones we’re providing” type. We might want a better name though…

function withExpander<TWrappedComponentProps extends ExpanderProps>(WrappedComponent: ComponentType<TWrappedComponentProps>) {
  type WrappedComponentPropsExceptProvided = Exclude<keyof TWrappedComponentProps, keyof ExpanderProps>;
  // => 'title' | 'className'
  type ForwardedProps = Pick<TWrappedComponentProps, WrappedComponentPropsExceptProvided>;
  // => { title: string; className?: string }

  //...
}

Now that we know what our forwarded props should look like we can specify those on the component.

return class WithExpander extends PureComponent<
  ForwardedProps,
  { isExpanded: boolean }
> {
  render() {
    return (
      <WrappedComponent
        {...this.props}
        {...this.state}
        toggleExpanded={this.toggleExpanded}
      />
    );
  }

  //...
};

the additional spread of this.props onto WrappedComponent copies the externally-specified props onto the wrapped component

Now we can use our enhanced component as intended.

const EnhancedComponent = withExpander(ExpanderComponent);

const usage = (
  <EnhancedComponent title="title">
    <div>...</div>
  </EnhancedComponent>
);

As a bonus, TypeScript gives us Requirement 4 for free! Type inference means that we don’t have to specify any types explicitly in our usage of this HoC but we still have full type safety. Try removing title or adding an unknown property and your IDE will light up with compilation errors – thanks TypeScript! ✨

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Post-Logout Redirect with ASP.NET Core and ADFS 2016

Redirect after logout

When a user logs out from your app you have the option to log them out of the provider as well by redirecting the browser to the logout endpoint. By default this means that the user will end up sat on your providers “You have signed out” page – not brilliant.

You can, however, tell your provider to redirect back to your app once they’re done with logout by specifying a post_logout_redirect_uri.

For ASP.NET Core Identity you can specify this redirection as a parameter on the SignOutResult.

[Route("auth")]
public class AuthController
{
  [HttpPost("logout")]
  public IActionResult Logout() => new SignOutResult(
    OpenIdConnectDefaults.AuthenticationScheme,
    new AuthenticationProperties { RedirectUri = Url.Action(nameof(LogoutSuccess))});

  [HttpGet("logoutSuccess")]
  public IActionResult LogoutSuccess() => View();
}

Useless ADFS error messages

For ADFS 2016 you need to do a little bit more than just set the redirect URL. On first inspection you can see that the above will set the parameter in the ADFS URL but ADFS will silently ignore it and your user will sit forever on the ADFS sign-out page.

Digging into the event logs you will find the following error message:

The specified redirect URL did not match any of the OAuth client’s redirect URIs. The logout was successful but the client will not be redirected

If you’re unlucky, this wonderfully-misleading error message can take you down a rabbit hole of further configuration. It’s a shame, then, that no-one thought to expose something a little more accurate:

That redirect looks good but you need to specify id_token_hint or we’ll ignore you

Thanks ADFS!

Sending ID Token Hint

To be fair to ADFS, sending an id_token_hint is recommended by the spec. This parameter needs to be set to the id_token that was sent to your app when the user first logged in; provide this value and ADFS will happily redirect back to your app.

The only problem here is that you probably don’t still have that id_token. ASP.NET Core Identity uses the identity information to create an auth cookie and then (by default) discards it.

Happily, the OpenIdConnectOptions exposes a SaveTokens property to persist the received token to the auth cookie. Even better: the OIDC logout mechanism will automatically pick this up once enabled so you should be good to go as soon as you set the flag:

public class Startup {
  public void ConfigureServices(IServiceCollection services)
  {
    services.AddAuthentication()
      .AddOpenIdConnect(options => {
        options.SaveTokens = true;
        //...set other options
      });

    //...other service config
  }
}

This does have one important downside though: you’re now storing much more information in your auth cookie and that adds extra data to every client request, maybe even doubling the cookie size.

Whether or not this is a problem for your app is another decision – at least your logout redirect works now!