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Basic Concepts and Terminology

This page introduces the basic concepts and terminology used in the @tanstack/react-form library. Familiarizing yourself with these concepts will help you better understand and work with the library.

Form Options

You can create options for your form so that it can be shared between multiple forms by using the formOptions function.

Example:

const formOpts = formOptions({
  defaultValues: {
    firstName: '',
    lastName: '',
    hobbies: [],
  } as Person,
})

Form Instance

A Form Instance is an object that represents an individual form and provides methods and properties for working with the form. You create a form instance using the useForm hook provided by the form options. The hook accepts an object with an onSubmit function, which is called when the form is submitted.

const form = useForm({
  ...formOpts,
  onSubmit: async ({ value }) => {
    // Do something with form data
    console.log(value)
  },
})

You may also create a form instance without using formOptions by using the standalone useForm API:

const form = useForm({
  onSubmit: async ({ value }) => {
    // Do something with form data
    console.log(value)
  },
  defaultValues: {
    firstName: '',
    lastName: '',
    hobbies: [],
  } as Person,
})

Field

A Field represents a single form input element, such as a text input or a checkbox. Fields are created using the form.Field component provided by the form instance. The component accepts a name prop, which should match a key in the form's default values. It also accepts a children prop, which is a render prop function that takes a field object as its argument.

Example:

<form.Field
  name="firstName"
  children={(field) => (
    <>
      <input
        value={field.state.value}
        onBlur={field.handleBlur}
        onChange={(e) => field.handleChange(e.target.value)}
      />
      <FieldInfo field={field} />
    </>
  )}
/>

If you run into issues handing in children as props, make sure to check your linting rules.

Example (ESLint):

  "rules": {
    "react/no-children-prop": [
      true,
      {
        "allowFunctions": true
      }
    ],
  }

Field State

Each field has its own state, which includes its current value, validation status, error messages, and other metadata. You can access a field's state using the field.state property.

Example:

const {
  value,
  meta: { errors, isValidating },
} = field.state

There are three field states that can be useful to see how the user interacts with a field: A field is "touched" when the user clicks/tabs into it, "pristine" until the user changes value in it, and "dirty" after the value has been changed. You can check these states via the isTouched, isPristine and isDirty flags, as seen below.

const { isTouched, isPristine, isDirty } = field.state.meta

Field states

Important note for users coming from React Hook Form: the isDirty flag in TanStack/form is different from the flag with the same name in RHF. In RHF, isDirty = true, when the form's values are different from the original values. If the user changes the values in a form, and then changes them again to end up with values that match the form's default values, isDirty will be false in RHF, but true in TanStack/form. The default values are exposed both on the form's and the field's level in TanStack/form (form.options.defaultValues, field.options.defaultValue), so you can write your own isDefaultValue() helper if you need to emulate RHF's behavior.`

Field API

The Field API is an object passed to the render prop function when creating a field. It provides methods for working with the field's state.

Example:

<input
  value={field.state.value}
  onBlur={field.handleBlur}
  onChange={(e) => field.handleChange(e.target.value)}
/>

Validation

@tanstack/react-form provides both synchronous and asynchronous validation out of the box. Validation functions can be passed to the form.Field component using the validators prop.

Example:

<form.Field
  name="firstName"
  validators={{
    onChange: ({ value }) =>
      !value
        ? 'A first name is required'
        : value.length < 3
          ? 'First name must be at least 3 characters'
          : undefined,
    onChangeAsync: async ({ value }) => {
      await new Promise((resolve) => setTimeout(resolve, 1000))
      return value.includes('error') && 'No "error" allowed in first name'
    },
  }}
  children={(field) => (
    <>
      <input
        value={field.state.value}
        onBlur={field.handleBlur}
        onChange={(e) => field.handleChange(e.target.value)}
      />
      <FieldInfo field={field} />
    </>
  )}
/>

Validation with Standard Schema Libraries

In addition to hand-rolled validation options, we also support the Standard Schema specification.

You can define a schema using any of the libraries implementing the specification and pass it to a form or field validator.

Supported libraries include:

import { z } from 'zod'

const userSchema = z.object({
  age: z.number().gte(13, 'You must be 13 to make an account'),
})

function App() {
  const form = useForm({
    defaultValues: {
      age: 0,
    },
    validators: {
      onChange: userSchema,
    },
  })
  return (
    <div>
      <form.Field
        name="age"
        children={(field) => {
          return <>{/* ... */}</>
        }}
      />
    </div>
  )
}

Reactivity

@tanstack/react-form offers various ways to subscribe to form and field state changes, most notably the useStore(form.store) hook and the form.Subscribe component. These methods allow you to optimize your form's rendering performance by only updating components when necessary.

Example:

const firstName = useStore(form.store, (state) => state.values.firstName)
//...
<form.Subscribe
  selector={(state) => [state.canSubmit, state.isSubmitting]}
  children={([canSubmit, isSubmitting]) => (
    <button type="submit" disabled={!canSubmit}>
      {isSubmitting ? '...' : 'Submit'}
    </button>
  )}
/>

It is important to remember that while the useStore hook's selector prop is optional, it is strongly recommended to provide one, as omitting it will result in unnecessary re-renders.

// Correct use
const firstName = useStore(form.store, (state) => state.values.firstName)
const errors = useStore(form.store, (state) => state.errorMap)
// Incorrect use
const store = useStore(form.store)

Note: The usage of the useField hook to achieve reactivity is discouraged since it is designed to be used thoughtfully within the form.Field component. You might want to use useStore(form.store) instead.

Listeners

@tanstack/react-form allows you to react to specific triggers and "listen" to them to dispatch side effects.

Example:

<form.Field
  name="country"
  listeners={{
    onChange: ({ value }) => {
      console.log(`Country changed to: ${value}, resetting province`)
      form.setFieldValue('province', '')
    },
  }}
/>

More information can be found at Listeners

Array Fields

Array fields allow you to manage a list of values within a form, such as a list of hobbies. You can create an array field using the form.Field component with the mode="array" prop.

When working with array fields, you can use the fields pushValue, removeValue, swapValues and moveValue methods to add, remove, and swap values in the array.

Example:

<form.Field
  name="hobbies"
  mode="array"
  children={(hobbiesField) => (
    <div>
      Hobbies
      <div>
        {!hobbiesField.state.value.length
          ? 'No hobbies found.'
          : hobbiesField.state.value.map((_, i) => (
              <div key={i}>
                <form.Field
                  name={`hobbies[${i}].name`}
                  children={(field) => {
                    return (
                      <div>
                        <label htmlFor={field.name}>Name:</label>
                        <input
                          id={field.name}
                          name={field.name}
                          value={field.state.value}
                          onBlur={field.handleBlur}
                          onChange={(e) => field.handleChange(e.target.value)}
                        />
                        <button
                          type="button"
                          onClick={() => hobbiesField.removeValue(i)}
                        >
                          X
                        </button>
                        <FieldInfo field={field} />
                      </div>
                    )
                  }}
                />
                <form.Field
                  name={`hobbies[${i}].description`}
                  children={(field) => {
                    return (
                      <div>
                        <label htmlFor={field.name}>Description:</label>
                        <input
                          id={field.name}
                          name={field.name}
                          value={field.state.value}
                          onBlur={field.handleBlur}
                          onChange={(e) => field.handleChange(e.target.value)}
                        />
                        <FieldInfo field={field} />
                      </div>
                    )
                  }}
                />
              </div>
            ))}
      </div>
      <button
        type="button"
        onClick={() =>
          hobbiesField.pushValue({
            name: '',
            description: '',
            yearsOfExperience: 0,
          })
        }
      >
        Add hobby
      </button>
    </div>
  )}
/>

Reset Buttons

When using <button type="reset"> in conjunction with TanStack Form's form.reset(), you need to prevent the default HTML reset behavior to avoid unexpected resets of form elements (especially <select> elements) to their initial HTML values. Use event.preventDefault() inside the button's onClick handler to prevent the native form reset.

Example:

<button
  type="reset"
  onClick={(event) => {
    event.preventDefault()
    form.reset()
  }}
>
  Reset
</button>

Alternatively, you can use <button type="button"> to prevent the native HTML reset.

<button
  type="button"
  onClick={() => {
    form.reset()
  }}
>
  Reset
</button>

These are the basic concepts and terminology used in the @tanstack/react-form library. Understanding these concepts will help you work more effectively with the library and create complex forms with ease.