Making reusable components is one of the best things about React. Developers write less duplicate code, and our sites and apps can have a more consistent UI. This is good!

Making content clear and logical to both screen readers and web parsers is also a good thing. Among other things, that means that heading elements (h1 - h6) need to appear in order. To learn more about why this matters, read Fen Slattery's excellent article on HTML headings.

Both of those things are important, but they don't always work together well. Working with components introduces the risk of disorganized headings. A component may be used in two places, with each page hierarcy demanding different heading levels. And as a project grows in size, a change to a component in one place may have unintended consequences in others.

What might that look like? Something like this:

const Banner = ({ headingText, description }) => (
  <div>
    <h1>{headingText}</h1>
    <p>{description}</p>
  </div>
)

That component could be a problem if you ever want to use it elsewhere in your app. Let's say you use the Banner on one page, and it has your h1. But what about later, when you want to use it halfway down a different page? One where the h1 element already exists? An h2 or h3 might be the right choice in that case. Whether you've got a banner, a card, or any other piece of UI that might need a heading, you should make it flexible.

How do we do that? React magic! Let's make a reusable heading component that can accept props for the heading level it should render. Then we can use our flexible heading component in our Banner.

A reusable JavaScript header

If you search the web for flexible react heading components, you might find something like this:

// a JavaScript flexible heading element
const JsHeading = ({ headingLevel }) => {
  const Heading = headingLevel
  return <Heading>{children}</Heading>
}

// our reusable banner
const Banner = ({ headingText, description }) => (
  <div>
    <JsHeading headingLevel="h2">{headingText}</JsHeading>
    <p>{description}</p>
  </div>
)

That's great... for regular JavaScript. In fact, Suzanne Aitchison has a great post on this as well. If you're not using TypeScript, I'd recommend reading her article. She ends with some valuable ideas on how to prevent unwanted behavior.

But what about TypeScript?

The title of this article is 'Reusable, accessible headings in React with TypeScript' - and the code above won't work in TypeScript. Even if you try adding explicit types, you won't get too far.

You might try casting your input, but I wouldn't recommend it. You'll end up casting to unknown in between and it's just gross. Besides, why cast when you can properly type everything to begin with? But, what types do we need?

You might think we should pass an heading element directly as a prop. I'm afraid we can't do that, Dave. You'll get an error if you try to directly pass an HTMLHeadingElement as a prop. The most flexible type you can pass in is React.ElementType. Let's take a look:

// extend your interface to be sure your heading element can have stuff like a className and children
interface HeadingProps extends React.HTMLAttributes<HTMLHeadingElement> {
  headingLevel: React.ElementType
}

const TsHeading: React.FC<HeadingProps> = ({
  headingLevel,
  children,
  className,
}) => {
  const Heading = headingLevel
  return <Heading className={className}>{children}</Heading>
}

// our reusable banner
const Banner: React.FC<{ headingText: string; description: string }> = ({
  headingText,
  description,
}) => (
  <div>
    <TsHeading headingLevel="h2">{headingText}</TsHeading>
    <p>{description}</p>
  </div>
)

Cool! Great! This works, and it renders just fine. Except... you can now pass in any element to your <Heading /> element.

const Banner: React.FC<{ headingText: string; description: string }> = ({
  headingText,
  description,
}) => (
  <div>
    <TsHeading headingLevel="table">{headingText}</TsHeading>
    <p>{description}</p>
  </div>
)

We can pass in a table and it won't give us an error - in fact, it will render to the page as <table>Whatever text you passed</table>.

Just the headings, please

So what's a safe way to ensure you only pass heading elements to your reusable <Heading />? This one took me a while to get right. I won't bore you with all my trials, tribulations, and failures - here's what I found that works.

// the interface needs to explicitly declare which strings are safe to pass
interface HeadingProps extends React.HTMLAttributes<HTMLHeadingElement> {
  headingLevel: "h1" | "h2" | "h3" | "h4" | "h5" | "h6" | "p"
}

const TsHeading = ({
  headingLevel = "p",
  children,
  className,
}: HeadingProps) => {
  const Heading = ({ ...props }: React.HTMLAttributes<HTMLHeadingElement>) =>
    React.createElement(headingLevel, props, children)

  return <Heading className={className}>{children}</Heading>
}

const Banner: React.FC<{ headingText: string; description: string }> = ({
  headingText,
  description,
}) => (
  <div>
    <TsHeading headingLevel="h2">{headingText}</TsHeading>
    <p>{description}</p>
  </div>
)

So what's going on here? Two things:

1. we need to explicitly tell the interface which strings (representing html elements) we want to accept. You might notice I've included "p" - this is a fallback, in case you want to use this element without a heading. You can adapt this to match your needs!
2. Our Heading just got more complex. Because we can't directly pass an HTML heading element to a React component as a prop, we instead need to pass a (valid) string, and use that in React's createElement method. React.createElement is actually how JSX works under the hood, but that's a totally different topic.

And that's it! Now you can use your heading component in any other reusable, composable component. Go forth and create accessible websites!