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TanStackForm
Guides

Basic Concepts and Terminology

This page introduces the basic concepts and terminology used in the @tanstack/svelte-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:

ts
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 createForm function. The function accepts an object with an onSubmit function, which is called when the form is submitted.

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

You may also create a form instance without using formOptions:

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

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:

svelte
<form.Field name="firstName">
  {#snippet children(field)}
    <input
      name={field.name}
      value={field.state.value}
      onblur={field.handleBlur}
      oninput={(e) => field.handleChange(e.target.value)}
    />
  {/snippet}
</form.Field>

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:

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

There are four states in the metadata that can be useful to see how the user interacts with a field:

  • "isTouched", after the user changes the field or blurs the field
  • "isDirty", after the field's value has been changed, even if it's been reverted to the default. Opposite of isPristine
  • "isPristine", until the user changes the field value. Opposite of isDirty
  • "isBlurred", after the field has been blurred
ts
const { isTouched, isDirty, isPristine, isBlurred } = field.state.meta

Field states

Understanding 'isDirty' in Different Libraries

Non-Persistent dirty state

  • Libraries: React Hook Form (RHF), Formik, Final Form.
  • Behavior: A field is 'dirty' if its value differs from the default. Reverting to the default value makes it 'clean' again.

Persistent dirty state

  • Libraries: Angular Form, Vue FormKit.
  • Behavior: A field remains 'dirty' once changed, even if reverted to the default value.

We have chosen the persistent 'dirty' state model. To also support a non-persistent 'dirty' state, we introduce an additional flag:

  • "isDefaultValue", whether the field's current value is the default value
ts
const { isDefaultValue, isTouched } = field.state.meta

// The following line will re-create the non-Persistent `dirty` functionality.
const nonPersistentIsDirty = !isDefaultValue

Field states extended

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:

svelte
<input
  name={field.name}
  value={field.state.value}
  onblur={field.handleBlur}
  oninput={(e) => field.handleChange(e.target.value)}
/>

Validation

@tanstack/svelte-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:

svelte
<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'
    },
  }}
>
  {#snippet children(field)}
    <input
      name={field.name}
      value={field.state.value}
      onBlur={field.handleBlur}
      onInput={(e) => field.handleChange(e.target.value)}
    />
    <p>{field.state.meta.errors[0]}</p>
  {/snippet}
</form.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:

svelte
<script>
  import { z } from 'zod'

  // ...
</script>

<form.Field
  name="firstName"
  validators={{
    onChange: z.string().min(3, 'First name must be at least 3 characters'),
    onChangeAsyncDebounceMs: 500,
    onChangeAsync: z.string().refine(
      async (value) => {
        await new Promise((resolve) => setTimeout(resolve, 1000))
        return !value.includes('error')
      },
      {
        message: 'No "error" allowed in first name',
      },
    ),
  }}
>
  {#snippet children(field)}
    <input
      name={field.name}
      value={field.state.value}
      onBlur={field.handleBlur}
      onInput={(e) => field.handleChange(e.target.value)}
    />
    <p>{field.state.meta.errors[0]}</p>
  {/snippet}
</form.Field>

Reactivity

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

Example:

svelte
<script>
  //...
  const firstName = form.useSelector((state) => state.values.firstName)
</script>

<form.Subscribe
  selector={(state) => ({
    canSubmit: state.canSubmit,
    isSubmitting: state.isSubmitting,
  })}
>
  {#snippet children(state)}
    <button type="submit" disabled={!state.canSubmit}>
      {state.isSubmitting ? '...' : 'Submit'}
    </button>
  {/snippet}
</form.Subscribe>

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, swap, and move a value from one index to another within the array, respectively. Additional helper methods such as insertValue, replaceValue, and clearValues are also available for inserting, replacing, and clearing array values.

Example:

svelte
<form.Field name="hobbies" mode="array">
  {#snippet children(hobbiesField)}
    <div>
      Hobbies
      <div>
        {#each hobbiesField.state.value as _, i}
            <div>
              <form.Field name={`hobbies[${i}].name`}>
                {#snippet children(field)}
                  <div>
                    <label for={field.name}>Name:</label>
                    <input
                      id={field.name}
                      name={field.name}
                      value={field.state.value}
                      onblur={field.handleBlur}
                      onchange={(e) => field.handleChange(e.target.value)}
                    />
                    <button
                      type="button"
                      onclick={() => hobbiesField.removeValue(i)}
                    >
                      X
                    </button>
                  </div>
                {/snippet}
              </form.Field>
              <form.Field name={`hobbies[${i}].description`}>
                {#snippet children(field)}
                    <div>
                      <label for={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)}
                      />
                    </div>
                {/snippet}
              </form.Field>
            </div>
          {:else}
            No hobbies found.
          {/each}
      </div>
      <button
        type="button"
        onclick={() =>
          hobbiesField.pushValue({
            name: '',
            description: '',
            yearsOfExperience: 0,
          })
        }
      >
        Add hobby
      </button>
    </div>
  {/snippet}
</form.Field>

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

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Form Validation