README
react-accessible-headings
Why?
In order to make accessible web pages the W3C: WCAG, WAI say,
Skipping heading ranks can be confusing and should be avoided where possible: Make sure that a
<h2>
is not followed directly by an<h4>
, for example.
So an accessible app must have heading levels like this...
- H1
- H2
- H3
- H3
- H2
- H3
- H4
- H4
- H3
- H2
- H2
Not like this,
- H1
- H6
- H3
- H1 (there should only be a single H1!)
- H5
- H4
- H4
- H1
However as developers of React components it's hard to make components match this semantic hierarchy. We typically hardcode heading levels, like an <h2>
, or an <h3>
, into a component. This would limit its flexibility and make it harder to adhere to W3C WCAG.
By using react-accessible-headings
you can have components with flexible headings that fit the appropriate heading level, allowing you to more easily create accessible components, with headings that don't skip levels.
Could you instead write components that accept props
to set a heading level? Sure. But that requires manual maintenance of the hierarchy. Indenting becomes harder, and it's easier to make mistakes.
This library is 1 kilobyte (minified and compressed).
Usage
import React from "react";
import { Level, H } from "react-accessible-headings";
export default function () {
return (
<div>
<H>This will be a heading 1</H>
<Level>
<H>and this a Heading 2</H>
</Level>
</div>
);
}
API
All APIs have TypeScript types available.
component <H>
This component renders either <h1>
, <h2>
, <h3>
, <h4>
, <h5>
, or <h6>
based on how many <Level>
s were above it.
react-accessible-headings
tries to help you maintain valid heading hierarchies, so it considers it an application bug to render an <h7>
(the HTML spec only has 6 heading levels). This might happen if you have too many <Level>
s above it.
To help debug the error...
- In Development mode an exception will be thrown if attempting to render invalid levels such as
h7
. Fix the wrongly nested<Level>
elements above it. - In Production mode there's different behaviour. No exception will be thrown, but a message will printed via
console.error
. The heading will be rendered but the value will be clamped from 1-6 (so an attempt to render<h7>
will be rendered as an<h6>
).
All valid props / attributes for an HTML heading are also accepted.
Props: offset
: (Optional) this optional prop will override the default behaviour. The default behaviour is when you use <H>
without this prop it will render the current heading level depth. If instead you want to render the <H>
with a different offset
(number) then provide this prop.
See Examples: The 'Offset' Example for more.
component <Level>
Sets a new heading level depth, by incrementing the current heading level for all children using the <H>
component, or the useLevel
hook.
This component doesn't render anything except children
, so there's no wrapper element.
Props: value
: (Optional) this optional prop will override the default behaviour. The default behaviour is when you use <Level>
without this prop it will increment the heading level by 1
. If you want to increment by a different value
(number) that is not 1
then provide this value
prop. You probably shouldn't be using this.
In Development mode an exception will be thrown if attempting to set an invalid value such as 7
, because HTML only has h1-h6. In Production mode an error will be logged via console.error
.
context hook useLevel
If for some reason you'd like to inspect the current level
value then useLevel()
which will return a number (integer) from 1-6. (see Examples: The 'useLevel query' Example for more).
In Development mode an exception will be thrown if useLevel
resolves to an invalid heading level such as 7
. In Production mode an error will be logged via console.error
, and the value will be clamped from 1-6 (because 7
is an invalid heading level and it would be pointless to use that).
context LevelContext
The raw React Context. Note that the value may be undefined
in which case you should infer a level of 1
. No clamping of valid ranges of values occurs.
Unusual Feature: Detecting skipped headings
react-accessible-headings
tries to encourage correct heading levels by polling the DOM for accessibility errors.
This only occurs during Development mode, not in Production.
There are two types of errors that are checked
- Whether there are skipped heading levels. Ie,
<h1>
followed by an<h3>
; - Whether there are multiple
<h1>
s in the page (there should only be a single<h1>
).
An exception will be thrown if any of these errors occur.
Testing in Axe will also reveal this type of error.
The reason this was implemented by polling the DOM, rather than analysing the React VDOM (or something), is because only the real DOM knows the actual heading levels that screen readers will use for accessibility reasons. Pages could include headings outside of React apps that affect the heading level, so this library needs to poll the real DOM, but only in Development mode.
Further reading
Prior art
DocBook, the ill-fated XHTML 2, and HTML5's abandoned 'outline' had a very similar idea. Also check out the 2014 project html5-h.
References
WCAG 2: G141: Organizing a page using headings,
To facilitate navigation and understanding of overall document structure, authors should use headings that are properly nested (e.g., h1 followed by h2, h2 followed by h2 or h3, h3 followed by h3 or h4, etc.).
Axe: Heading levels should only increase by one
Ensure headings are in a logical order. For example, check that all headings are marked with
h1
throughh6
elements and that these are ordered hierarchically. For example, the heading level following anh1
element should be anh2
element, not anh3
element.
Axe: Page must contain a level-one heading
Generally, it is a best practice to ensure that the beginning of a page's main content starts with a h1 element, and also to ensure that the page contains only one h1 element.
#
JustificationsIs this library necessary? Could you avoid this library and perhaps make component props
that set the heading level, or use children
to set the heading? Sure, that works, but (arguably) that manual approach becomes a maintenance problem across a larger app. Across a whole app this alternative approach is easier to refactor and 'indent' heading levels arbitrarily without having to synchronise the correct heading level numbers across components.
#
The 'Card' ExampleImagine you have a hypothetical 'Card' component that is coded as,
export function Card({ children, heading }) {
return (
<div className="card">
<h3 className="card__heading">{heading}</h3>
{children}
</div>
);
}
But then you want to make the <h3>
configurable to make it either an <h2>
, <h3>
, or <h4>
.
You might refactor the code to support that feature like this,
export function Card({ children, heading, headingLevel }) {
return (
<div className="card">
{headingLevel === 2 ? (
<h2 className="card__heading">{heading}</h2>
) : headingLevel === 3 ? (
<h3 className="card__heading">{heading}</h3>
) : headingLevel === 4 ? (
<h4 className="card__heading">{heading}</h4>
) : null}
{children}
</div>
);
}
or more concisely,
export function Card({ children, heading, headingLevel }) {
const Heading = `H${headingLevel}`;
return (
<div className="card">
<Heading className="card__heading">{heading}</Heading>
{children}
</div>
);
}
...which is a confusingly indirect way of making a heading level, and it creates a maintenance burden on developers to know the correct level depth of a heading.
Alternatively, with react-accessible-headings
the implementation details of <Card>
can stay encapsulated and look like,
export function Card({ children, heading }) {
return (
<div className="card">
<H className="card__heading">{heading}</H>
{children}
</div>
);
}
And finally (for this example) let's consider another refactoring. If we want to add a new h2
to the page and lower every other heading it's now easy to add another <Level>
wrapper to indent everything and you're done. Much easier than updating lots of h*
numbers around the code to realign them all...
<H>Cards</H>
<Level>
<Card heading="my title">
<p>body</p>
</Card>
<Level>
<Card heading="my title2">
<p>body</p>
</Card>
</Level>
</Level>
So react-accessible-headings
is an alternative composition technique for page headings that may make it easier to refactor and reuse code. The <Level>
concept means you only need to think about whether it's a deeper level, without having to know the specific heading level number.
That all said, having a flexible heading level may be more abstract and confusing to some developers. It's an extra thing to learn, even though it is a simple concept. It may not be appropriate for some codebases.
#
The 'useLevel query' ExampleIf you want to programatically query the current level you can,
import { useLevel, H } from "react-accessible-headings";
export default function () {
const level = useLevel(); // level is a number (integer) from 1-6
return (
<div className={`heading--${level}`}>
<H>text</H>
</div>
);
}
#
The 'Offset' ExampleIf you want to have heading levels relative to the current level you can provide an offset
prop,
<div className="card">
<H className="card__heading">This will be the current heading level</H>
<H offset={1} className="card__sub-heading">
This will be one level deeper
</H>
{children}
</div>
which is a more concise way of writing this,
<div className="card">
<H className="card__heading">This will be the current heading level</H>
<Level>
<H className="card__sub-heading">This will be one level deeper</H>
</Level>
{children}
</div>
However <Level>
will establish a new deeper heading level context whereas offset
will not.