Simple form validation for React

Simple form validation for React

Simple form validation for React


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Simple form validation for React

  • Installation: npm install react-form-with-constraints
  • CDN:

Check the changelog for breaking changes and fixes between releases.

Introduction: what is HTML5 form validation?

⚠️ Client side validation is cosmetic, you should not rely on it to enforce security

  <label for="email">Email:</label>
  <input type="email" id="email" required>

input required input type="email"

The required HTML5 attribute specifies that the user must fill in a value, type="email" checks that the entered text looks like an email address.


What react-form-with-constraints brings

  • Minimal API and footprint
  • Unobtrusive: easy to adapt regular React code
  • HTML5 error messages personalization: <FieldFeedback when="valueMissing">My custom error message</FieldFeedback>
  • Custom constraints: <FieldFeedback when={value => ...}>
  • Warnings and infos: <FieldFeedback ... warning>, <FieldFeedback ... info>
  • Async validation
  • No dependency beside React (no Redux, MobX...)
  • Re-render only what's necessary
  • Easily extendable
  • Bootstrap 4 styling with npm package react-form-with-constraints-bootstrap4
  • Material-UI integration with npm package react-form-with-constraints-material-ui
  • Support for React Native with npm package react-form-with-constraints-native
  • ...
<input type="password" name="password"
       value={this.state.password} onChange={this.handleChange}
       required pattern=".{5,}" />
<FieldFeedbacks for="password">
  <FieldFeedback when="valueMissing" />
  <FieldFeedback when="patternMismatch">
    Should be at least 5 characters long
  <FieldFeedback when={value => !/\d/.test(value)} warning>
    Should contain numbers
  <FieldFeedback when={value => !/[a-z]/.test(value)} warning>
    Should contain small letters
  <FieldFeedback when={value => !/[A-Z]/.test(value)} warning>
    Should contain capital letters


How it works

The API works the same way as React Router v4:

  <Route exact path="/" component={Home} />
  <Route path="/news" component={NewsFeed} />

It is also inspired by AngularJS ngMessages.

If you had to implement validation yourself, you would end up with a global object that tracks errors for each field. react-form-with-constraints works similarly. It uses React context to share the FieldsStore object across FieldFeedbacks and FieldFeedback.


The API reads like this: "for field when constraint violation display feedback", example:

<FieldFeedbacks for="password">
  <FieldFeedback when="valueMissing" />
  <FieldFeedback when="patternMismatch">Should be at least 5 characters long</FieldFeedback>
for field "password"
  when constraint violation "valueMissing"    display <the HTML5 error message (*)>
  when constraint violation "patternMismatch" display "Should be at least 5 characters long"

(*) element.validationMessage

Async support works as follow:

<FieldFeedbacks for="username">
    promise={checkUsernameAvailability} /* Function that returns a promise */
    then={available => available ?
      <FieldFeedback key="1" info style={{color: 'green'}}>Username available</FieldFeedback> :
      <FieldFeedback key="2">Username already taken, choose another</FieldFeedback>
      // Why key=*? Needed otherwise React gets buggy when the user rapidly changes the field

Trigger validation:

function MyForm() {
  const form = useRef(null);

  async function handleChange(e) {
    const target =;

    // Validates only the given fields and returns Promise<Field[]>
    const fields = await form.current.validateFields(target);

  async function handleSubmit(e) {

    // Validates the non-dirty fields and returns Promise<Field[]>
    const fields = await form.current.validateForm();

    if (form.current.isValid()) console.log('The form is valid');
    else console.log('The form is invalid');

  return (
    <FormWithConstraints ref={form} onSubmit={handleSubmit} noValidate>
        required minLength={3}
      <FieldFeedbacks for="username">
        <FieldFeedback when="tooShort">Too short</FieldFeedback>
          then={available => available ?
            <FieldFeedback key="1" info style={{color: 'green'}}>Username available</FieldFeedback> :
            <FieldFeedback key="2">Username already taken, choose another</FieldFeedback>
        <FieldFeedback when="*" />
  • FieldFeedbacks

    • for: string => reference to a name attribute (e.g <input name="username">), should be unique to the current form
    • stop?: 'first' | 'first-error' | 'first-warning' | 'first-info' | 'no' => when to stop rendering FieldFeedbacks, by default stops at the first error encountered (FieldFeedbacks order matters)

    Note: you can place FieldFeedbacks anywhere, have as many as you want for the same field, nest them, mix them with FieldFeedback... Example:

    <input name="username" ... />
    <FieldFeedbacks for="username" stop="first-warning">
        <FieldFeedback ... />
        <Async ... />
        <FieldFeedbacks stop="first-info">
      <FieldFeedback ... />
      <Async ... />
    <FieldFeedbacks for="username" stop="no">
  • FieldFeedback

    • when?:
      • ValidityState as a string => HTML5 constraint violation name
      • '*' => matches any HTML5 constraint violation
      • 'valid' => displays the feedback only if the field is valid
      • (value: string) => boolean => custom constraint
    • error?: boolean => treats the feedback as an error (default)
    • warning?: boolean => treats the feedback as a warning
    • info?: boolean => treats the feedback as an info
    • children => what to display when the constraint matches; if missing, displays the HTML5 error message if any
  • Async<T> => Async version of FieldFeedback (similar API as react-promise)

    • promise: (value: string) => Promise<T> => a promise you want to wait for
    • pending?: React.ReactNode => runs when promise is pending
    • then?: (value: T) => React.ReactNode => runs when promise is resolved
    • catch?: (reason: any) => React.ReactNode => runs when promise is rejected
  • FormWithConstraints

    • validateFields(...inputsOrNames: Array<Input | string>): Promise<Field[]> => Should be called when a field changes, will re-render the proper FieldFeedbacks (and update the internal FieldsStore). Without arguments, all fields ($('[name]')) are validated.

    • validateFieldsWithoutFeedback(...inputsOrNames: Array<Input | string>): Promise<Field[]> => Validates only all non-dirty fields (won't re-validate fields that have been already validated with validateFields()), If you want to force re-validate all fields, use validateFields(). Might be renamed to validateNonDirtyFieldsOnly() or validateFieldsNotDirtyOnly() in the future?

    • validateForm(): Promise<Field[]> => Same as validateFieldsWithoutFeedback() without arguments, typically called before to submit the form. Might be removed in the future?

    • isValid(): boolean => should be called after validateFields(), validateFieldsWithoutFeedback() or validateForm(), indicates if the fields are valid

    • hasFeedbacks(): boolean => indicates if any of the fields have any kind of feedback

    • resetFields(...inputsOrNames: Array<Input | string>): Promise<Field[]> => Resets the given fields and re-render the proper FieldFeedbacks. Without arguments, all fields ($('[name]')) are reset.

    • Field =>

        name: string;
        validations: { // FieldFeedbackValidation[]
          key: number;
          type: 'error' | 'warning' | 'info' | 'whenValid';
          show: boolean | undefined;
        isValid: () => boolean
  • Input

    If you want to style <input>, use <Input> instead: it will add classes is-pending, has-errors, has-warnings, has-infos and/or is-valid on <input> when the field is validated.

    Example: <Input name="username" /> can generate <input name="username" class="has-errors has-warnings">

    FYI react-form-with-constraints-bootstrap4 and react-form-with-constraints-material-ui already style the fields to match their respective frameworks.

Browser support

react-form-with-constraints needs ValidityState which is supported by all modern browsers and IE >= 10. It also needs a polyfill such as core-js or @babel/polyfill to support IE <= 11, see React JavaScript Environment Requirements.

You can use HTML5 attributes like type="email", required, minlength...

<label htmlFor="email">Email</label>
<input type="email" name="email" id="email"
       value={} onChange={this.handleChange}
       required />
<FieldFeedbacks for="email">
  <FieldFeedback when="*" />

...and/or rely on when functions:

<label htmlFor="email">Email</label>
<input name="email" id="email"
       value={} onChange={this.handleChange} />
<FieldFeedbacks for="email">
  <FieldFeedback when={value => value.length === 0}>Please fill out this field.</FieldFeedback>
  <FieldFeedback when={value => !/\[email protected]\S+/.test(value)}>Invalid email address.</FieldFeedback>

In the last case you will have to manage translations yourself (see SignUp example).

How to consume the npm packages?

ESNext (currently ES2018) + ES modules

Files inside lib/ (package.json "module": "lib/index.js").

A recent browser or Node.js is required or you will need to transpile the react-form-with-constraints source code using Babel (or TypeScript tsc).

Several advantages:

  • The combine use of "sideEffects": false with "module": ... generates a smaller bundle thanks to tree shaking
  • You can transpile react-form-with-constraints source code with your Babel's preset-env and Browserslist configuration

For this to work, do not exclude node_modules from your webpack configuration, example:

// webpack.config.js
module: {
  rules: [
      test: /\.(js|jsx?)$/,

      // See [Enable babel-preset-env for node_modules that target newer Node versions](
      // See [Create React App 2.0: "You can now use packages written for latest Node versions without breaking the build"](
      // See ["If you have to exclude node_modules/, how do you get babel to polyfill/transform code in 3rd party code?"](
      // See [Compile dependencies with babel-preset-env](
      //exclude: /node_modules/,
      exclude: /\/core-js/,

      loader: 'babel-loader',
      options: {
        // See
        compact: false

You probably want to configure Babel with sourceType: 'unambiguous':

// babel.config.js
module.exports = {
  // See
  // See [Add Babel config sourceType: 'unambiguous' for dependencies](
  sourceType: 'unambiguous',

  presets: [
        useBuiltIns: 'usage'
  plugins: [

ES5 + CommonJS

Classic ES5 transpilation, files inside lib-es5/ (package.json "main": "lib-es5/index.js"). No tree shaking.

UMD (Universal Module Definition) + ES5

Files inside dist/. Typical use is with <script src="react-form-with-constraints.production.min.js"> inside your index.html.

A good use case is CodePen, files are generated by Rollup.


  • A type="hidden", readonly or disabled input won't trigger any HTML5 form constraint validation like required, see
  • With <input type="number"> it's better to use onInput instead of onChange, see

Github Repository