This post was originally published on my blog

Forms are an important part of website and app development. They are critical elements in terms of UX and accessibility because they are the parts that users actually interact with. Poorly designed forms are not only annoying for everyone, but if they are not properly marked up, they may also hinder the use of assistive technologies.

However, the implementation of a user-friendly form is complex, and if it includes validation by JavaScript, it can be burdensome for developers. In this article, I would like to introduce the implementation of a React component called FormControl that helps us appropriately markup the form fields.

The implementation in this article is largely based on MUI - FormControl.

Usage

If you want to use all options:

return (
  <FormControl labelText="Name" helperText="Tell us your name" error="Something is wrong">
    <TextInput required placeholder="John Doe" />
  </FormControl>
)

The following is the output HTML. The id is automatically published within the component and is properly linked to the label element. In addition, aria attributes such as aria-describedby are also assigned.

<div class="FormControl">
  <label for=":r1:" id=":r1:--label">Name</label>
  <div id=":r1:--helper-text">Tell us your name</div>
  <input
    id=":r1:"
    aria-describedby=":r1:--helper-text :r1:--error-text"
    aria-invalid="true"
    placeholder="John Doe"
  />
  <div id=":r1:--error-text">Something is wrong</div>
</div>

Here is a style example. We will not touch on CSS much in this article, but you can pass the error prop to add a class name, or use the &:has([required]) selector to display an asterisk for required items as a pseudo-element.

Firefox doesn't support :has() yet.

Implementation method

Approach

As a premise, it is required that the user input component can be flexibly passed to the TextInput part mentioned above. Not all form fields are input, so it should be loosely coupled.

This time, we will use the Context of React as an interface. FormControl will add appropriate markup to the user input element received as children, wrap it with Context.Provider, and pass data such as the automatically generated id. This allows the child element to consume the data by calling useContext.

Define Context

Let's define FormControlContextValue as follows:

import React from "react"

export type FormControlContextValue = {
  id: string | undefined
  labelId: string | undefined
  helperTextId: string | undefined
  errorTextId: string | undefined
  isError: boolean | string
}

export const FormControlContext = React.createContext<FormControlContextValue>({
  id: undefined,
  labelId: undefined,
  helperTextId: undefined,
  errorTextId: undefined,
  isError: false,
})

export function useFormControlContext() {
  return React.useContext(FormControlContext)
}

Implement FormControl

We generate a single id to pass to the input element using React.useId and add a suffix to other IDs. Other than being surrounded by FormControlContext.Provider, nothing is special.

import React from "react"

import { FormControlContext } from "./FormControlContext"

export type FormControlProps = {
  labelText: string
  helperText?: string
  error?: boolean | string
  children: React.ReactNode
}

export function FormControl({
  labelText,
  helperText: helperText,
  error = false,
  children,
}: FormControlProps) {
  const id = React.useId()
  const labelId = `${id}--label`
  const helperTextId = `${id}--helper-text`
  const errorTextId = `${id}--error-text`
  const ctxValue = {
    id,
    labelId,
    helperTextId,
    errorTextId,
    isError: !!error,
  }

  return (
    <FormControlContext.Provider value={ctxValue}>
      <div>
        <label htmlFor={id} id={labelId}>
          {labelText}
        </label>
        {helperText && <span id={helperText}>{helperText}</span>}
        {children}
        {typeof error === "string" && <span id={errorTextId}>{error}</span>}
      </div>
    </FormControlContext.Provider>
  )
}

Implement Input

We will first implement the generic Input component, rather than the TextInput component in the first example. By calling useFormControlContext(), you can use the values provided by the enclosing FormControl.

• aria-describedby can specify multiple IDs separated by spaces, so we pass both supplementary explanatory text and error messages.
• isError passed to aria-invalid becomes true when error is specified with a truthy value (true or a non-empty string). You can style erroneous state with CSS selectors like input[aria-invalid="true"].

import React from "react"

import { useFormControlContext } from "../FormControl/useFormControlContext"

type Props = JSX.IntrinsicElements["input"]

export const Input = React.forwardRef<HTMLInputElement, Props>((props, forwardedRef) => {
  const { id, helperTextId, errorTextId, isError } = useFormControlContext()
  return (
    <input
      id={id}
      aria-describedby={[helperTextId || "", errorTextId || ""].join(" ")}
      aria-invalid={isError}
      ref={forwardedRef}
      {...props}
    />
  )
})

Input.displayName = "Input"

In this implementation, extra spaces are added to aria-describedby, so it may be better to do some filtering.

Extend Input

You can use the Input component as is, or create extended components such as TextInput or Checkbox by specifying the type. It should be easy to integrate with external libraries by passing them id.

Other considerations

What if nested?

Since useContext refers to the value of the nearest Provider, there is no problem even if FormControl is nested. Therefore, it can be used as follows.

export function RadioGroup({ children }: RadioGroupProps) {
  const { labelId } = useFormControlContext()
  return (
    <div role="radiogroup" aria-labeledby={labelId}>
      {children}
    </div>
  )
}

return (
  <FormControl labelText="PPAP">
    <RadioGroup>
      <FormControl labelText="Pen">
        <Input type="radio" name="ppap" value="pen" />
      </FormControl>
      <FormControl labelText="Pineapple">
        <Input type="radio" name="ppap" value="pineapple" />
      </FormControl>
      <FormControl labelText="Apple">
        <Input type="radio" name="ppap" value="apple" />
      </FormControl>
    </RadioGroup>
  </FormControl>
)

Is it bad idea to enclose input by label?

Although it is certainly convenient to eliminate the management of IDs by putting input inside label, I think it is better to link with htmlFor if you want to support aria-describedby and behavior when nested. If those are not needed, it is also fine to simply surround it.

What are the other implementation methods besides Context?

It may be convenient to also support render props.

export type FormControlProps = {
  {...},
  children:
    | React.ReactNode
    | ((props: FormControlContextValue) => React.ReactNode);
};

function FormControl() {
  {...}
  return (
    {...}
    {typeof children === "function" ? children(ctxValue) : children}
    {...}
  )
}

function Component() {
  return (
    <FormControl labelText={labelText}>
      {({ id, helperTextId }) => (
        <input id={id} aria-describedby={helperTextId} />
      )}
    </FormControl>
  )
}

TODO

I think the FormControl implementation we did this time can solve most markup problems, but there are still things left to do such as styling and validation.

Styling

Not all labels and supplementary text will be displayed in the same place or appearance. For example, you should probably place radio buttons horizontally with the label. The appearance of FormControl should be able to be flexibly set from the outside.

Validation

Since FormControl does not do anything more than helping markup, I think it is quite compatible with React Hook Form. If you want to add client-side behavior, it is probably easier to separate responsibilities by overlaying it as a higher layer (rather than modifying FormControl).