Form Groups

When building a multi-stage form that has many stages, like so:

Form stepper

It's common for each step to have its own form. However, this complicates the form submission and validation process by requiring you to add complex logic.

Luckily, TanStack Form provides a way to build out sub-forms that make this kind of development trivial to implement: form.group(...).

Usage

To use a form group in TanStack Form, you'll create a TanStackFormController, then use its group directive like you would use its field directive:

import { LitElement, html } from 'lit'
import { customElement } from 'lit/decorators.js'
import { TanStackFormController } from '@tanstack/lit-form'

@customElement('my-form')
export class MyForm extends LitElement {
  #form = new TanStackFormController(this, {
    defaultValues: {
      step1: {
        name: '',
      },
      step2: {
        age: 0,
      },
    },
  })

  render() {
    return html`
      ${this.#form.group({ name: 'step1' }, (group) => {
        // `group` here has all of the form-like methods you'd expect like `deleteField` or `insertFieldValue`
        // ...
        return html``
      })}
    `
  }
}

This becomes much more useful when paired with external state to conditionally render a form group:

import { LitElement, html, nothing } from 'lit'
import { customElement, state } from 'lit/decorators.js'
import { TanStackFormController } from '@tanstack/lit-form'

@customElement('my-form')
export class MyForm extends LitElement {
  @state()
  private step = 0

  #form = new TanStackFormController(this, {
    defaultValues: {
      step1: {
        name: '',
      },
      step2: {
        age: 0,
      },
    },
  })

  render() {
    return html`
      ${this.step === 0
        ? this.#form.group(
            {
              name: 'step1',
              onGroupSubmit: () => {
                // We can move the step forward when validation passes
                this.step++
              },
              onGroupSubmitInvalid: () => {
                // Or handle invalid submissions, just like a top-level form
              },
              onSubmitMeta: {} as SomeType,
            },
            (group) => html`
              <form
                @submit=${(e: Event) => {
                  e.preventDefault()
                  e.stopPropagation()
                  // Use `group.handleSubmit()` to submit the sub-form, but not the parent form
                  group.handleSubmit()
                }}
              >
                ${this.#form.field(
                  { name: 'step1.name' },
                  (field) => html`<!-- ... -->`,
                )}
              </form>
            `,
          )
        : nothing}
      ${this.step === 1
        ? this.#form.group(
            {
              name: 'step2',
              onGroupSubmit: () => {
                // Then, use `this.#form.api.handleSubmit()` to submit the entire form
                this.#form.api.handleSubmit()
              },
            },
            (group) => html`
              <form
                @submit=${(e: Event) => {
                  e.preventDefault()
                  e.stopPropagation()
                  group.handleSubmit()
                }}
              >
                ${this.#form.field(
                  { name: 'step2.age' },
                  (field) => html`<!-- ... -->`,
                )}
              </form>
            `,
          )
        : nothing}
    `
  }
}

When you split each step into its own custom element, pass the TanStackFormController as a property and type it with getFormType. Because Lit child elements that receive a controller by property do not automatically re-render when the controller state changes, subscribe to form.api.store with TanStackStoreSelector in the child element.

Form Group Validation

Form groups have a distinct validation procedure that we think makes sense for sub-forms:

Form groups can have their own validation:

${this.#form.group(
  { name: 'step1', validators: { onChange: () => 'Error' } },
  (group) => html`
    <!-- group.state.meta.errorMap // {onChange: "Error" | undefined} -->
    <!-- group.state.meta.errors // ("Error")[] -->
  `,
)}

Can set errors on sub-fields:

${this.#form.group(
  {
    name: 'step1',
    validators: {
      onChange: ({ value, groupApi }) => ({
        group: value.name === 'error' ? 'Group error' : undefined,
        fields: {
          // Must use the name of the field relative to the form group as the error key,
          // to stay consistent with how standard schema works with form groups
          name: value.name === 'error' ? 'Field error' : undefined,
        },
      }),
    },
  },
  (group) => html`<!-- ... -->`,
)}

And can even accept standard schemas:

${this.#form.group(
  {
    name: 'step1',
    validators: {
      onChange: z.object({
        name: z.string().min(2),
      }),
    },
  },
  (group) => html`<!-- ... -->`,
)}

The reason we don't use the full path names for fields is so that you can compose your schemas like so:
ts
const step1Schema = z.object({
  name: z.string().min(2),
})

const schema = z.object({
  step1: step1Schema,
  step2: step2Schema,
})
And pass the step1Schema to a form group and schema to the parent form. That way, partially validated data will still flag errors if the group is bypassed.

Dynamic Group Validation

If you want to use dynamic validation (onDynamic) with a form group, do not rely on the onDynamic validator passed to TanStackFormController:

#form = new TanStackFormController(this, {
  validationLogic: revalidateLogic(),
  validators: {
    // This validator will not run `onChange` when a sub-form is submitted;
    // it will only run `onChange` when the form itself is submitted.
    onDynamic: schema,
  },
})

Instead, pass your sub-schema for the group to the onDynamic validation of the group itself:

${this.#form.group(
  { name: 'step1', validators: { onDynamic: step1Schema } },
  (group) => html`<!-- ... -->`,
)}

It will treat group.submissionAttempts as the way to change what validator is run before/after submit.

Form Group State

Just like you're able to access group.state.meta.errors, you're also able to access the group's value using group.state.value. Likewise, here are some valuable properties you can access in the group.state.meta:

group.state.meta.isFieldsValid: true when the field-level validators have no errors
group.state.meta.isGroupValid: true when the group-level validators have no errors
group.state.meta.isValid: true when both the field-level and group-level validators have no errors
group.state.meta.isSubmitting: true when the group is in the process of being submitted