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    Simple form validation for React

    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>
      <button type="submit">Submit</button>

    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:

      <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({ target }) {
        // Validates only the given fields and returns Promise<Field[]>
        await form.current.validateFields(target);
      async function handleSubmit(e) {
        // Validates the non-dirty fields and returns Promise<Field[]>
        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="*" />

    Important note:

    If a field (i.e an <input>) does not have a matching FieldFeedbacks, the library won't known about this field (and thus won't perform validation). The field name should match FieldFeedbacks.for:

    <input name="MY_FIELD" ...>
    <FieldFeedbacks for="MY_FIELD">

    • 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>): 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 11. It also needs a polyfill such as core-js 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 => !/\S+@\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: /\.jsx?$/,
          //exclude: /node_modules/,
          // [Babel should not transpile core-js](
          exclude: /\/core-js/,
          loader: 'babel-loader'
    // babel.config.js
    module.exports = {
      presets: [
            useBuiltIns: 'entry',
            corejs: 3
      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.



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