React Example: Simple

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import * as React from 'react'
import { createRoot } from 'react-dom/client'

import { TanStackDevtools } from '@tanstack/react-devtools'
import { formDevtoolsPlugin } from '@tanstack/react-form-devtools'
import { useForm } from '@tanstack/react-form'

import type { AnyFieldApi } from '@tanstack/react-form'

function FieldInfo({ field }: { field: AnyFieldApi }) {
  return (
    <>
      {field.meta.isTouched && field.meta.isInvalid ? (
        <em>{field.errors.map((error) => error.message).join(',')}</em>
      ) : null}
      {field.meta.isValidating ? 'Validating...' : null}
    </>
  )
}

export default function App() {
  const form = useForm({
    defaultValues: {
      firstName: '',
      lastName: '',
    },
    onSubmit: async ({ value }) => {
      // Do something with form data
      console.log(value)
    },
  })

  return (
    <div>
      <h1>Simple Form Example</h1>
      <form
        onSubmit={(e) => {
          e.preventDefault()
          e.stopPropagation()
          form.handleSubmit()
        }}
      >
        <div>
          {/* A type-safe field component*/}
          <form.Field
            name="firstName"
            validators={[
              {
                run: ({ value }) =>
                  !value
                    ? 'A first name is required'
                    : value.length < 3
                      ? 'First name must be at least 3 characters'
                      : undefined,
                triggers: ['change'],
              },
              {
                run: async ({ value }) => {
                  await new Promise((resolve) => setTimeout(resolve, 1000))
                  return (
                    value.includes('error') &&
                    'No "error" allowed in first name'
                  )
                },
                triggers: ['change'],
                triggerDebounceMs: 500,
              },
            ]}
            children={(field) => {
              // Avoid hasty abstractions. Render props are great!
              return (
                <>
                  <label htmlFor={field.name}>First Name:</label>
                  <input
                    id={field.name}
                    name={field.name}
                    value={field.value}
                    onBlur={field.handleBlur}
                    onChange={(e) => field.handleChange(e.target.value)}
                  />
                  <FieldInfo field={field} />
                </>
              )
            }}
          />
        </div>
        <div>
          <form.Field
            name="lastName"
            children={(field) => (
              <>
                <label htmlFor={field.name}>Last Name:</label>
                <input
                  id={field.name}
                  name={field.name}
                  value={field.value}
                  onBlur={field.handleBlur}
                  onChange={(e) => field.handleChange(e.target.value)}
                />
                <FieldInfo field={field} />
              </>
            )}
          />
        </div>
        <form.Subscribe
          selector={(state) => [state.canSubmit, state.isSubmitting]}
          children={([canSubmit, isSubmitting]) => (
            <>
              <button type="submit" disabled={!canSubmit}>
                {isSubmitting ? '...' : 'Submit'}
              </button>
              <button
                type="reset"
                onClick={(e) => {
                  // Avoid unexpected resets of form elements (especially <select> elements)
                  e.preventDefault()
                  form.reset()
                }}
              >
                Reset
              </button>
            </>
          )}
        />
      </form>
    </div>
  )
}

const rootElement = document.getElementById('root')!

createRoot(rootElement).render(
  <React.StrictMode>
    <App />

    <TanStackDevtools
      config={{ hideUntilHover: true }}
      plugins={[formDevtoolsPlugin()]}
    />
  </React.StrictMode>,
)