formisch-usage（形式用法）

Formisch Usage

This skill helps AI agents work effectively with Formisch, the schema-based, headless form library for modern frameworks.

When to Use This Skill

• When the user asks about form handling with Formisch
• When managing form state and validation
• When working with React, Vue, Solid, Preact, Svelte, or Qwik forms
• When integrating Valibot schemas with forms

Introduction

Formisch is a schema-based, headless form library that works across multiple frameworks. Key highlights:

• Small bundle size — Starting at ~2.5 kB
• Schema-based validation — Uses Valibot for type-safe validation
• Headless design — You control the UI completely
• Type safety — Full TypeScript support with autocompletion
• Framework-native — Native performance for each supported framework

Supported Frameworks

Installation

1. Install Valibot (peer dependency)

npm install valibot

2. Install Formisch for your framework

npm install @formisch/react   # React
npm install @formisch/vue     # Vue
npm install @formisch/solid   # SolidJS
npm install @formisch/preact  # Preact
npm install @formisch/svelte  # Svelte
npm install @formisch/qwik    # Qwik

Core Concepts

Schema-First Design

Every form starts with a Valibot schema. Types are automatically inferred from the schema.

import * as v from "valibot";

const LoginSchema = v.object({
  email: v.pipe(
    v.string("Please enter your email."),
    v.nonEmpty("Please enter your email."),
    v.email("The email address is badly formatted."),
  ),
  password: v.pipe(
    v.string("Please enter your password."),
    v.nonEmpty("Please enter your password."),
    v.minLength(8, "Your password must have 8 characters or more."),
  ),
});

Form Store

The form store manages all form state. Access it via the framework-specific hook/primitive.

Form Store Properties:

• isSubmitting — Form is currently being submitted
• isSubmitted — Form has been successfully submitted
• isValidating — Validation is in progress
• isTouched — At least one field has been touched
• isDirty — At least one field differs from initial value
• isValid — All fields pass validation
• errors — Root-level validation errors

Field Store

Each field has its own reactive store with:

• path — Path array to the field
• input — Current field value
• errors — Field-specific errors
• isTouched — Field has been focused and blurred
• isDirty — Field value differs from initial value
• isValid — Field passes validation
• props — Props to spread onto input elements
• onChange (React) / onInput (other frameworks) — Sets the field input value programmatically. Use this when the field cannot be connected to a native HTML element.

Dirty Tracking

Formisch tracks two inputs per field:

• Initial input — Baseline for dirty tracking (server state)
• Current input — What the user is editing (client state)

isDirty becomes true when current input differs from initial input.

Framework Examples

React Example

import { Field, Form, useForm } from "@formisch/react";
import type { SubmitHandler } from "@formisch/react";
import * as v from "valibot";

const LoginSchema = v.object({
  email: v.pipe(v.string(), v.email()),
  password: v.pipe(v.string(), v.minLength(8)),
});

export default function LoginPage() {
  const loginForm = useForm({
    schema: LoginSchema,
  });

  const handleSubmit: SubmitHandler<typeof LoginSchema> = (output) => {
    console.log(output); // { email: string, password: string }
  };

  return (
    <Form of={loginForm} onSubmit={handleSubmit}>
      <Field of={loginForm} path={["email"]}>
        {(field) => (
          <div>
            <input {...field.props} value={field.input} type="email" />
            {field.errors && <div>{field.errors[0]}</div>}
          </div>
        )}
      </Field>
      <Field of={loginForm} path={["password"]}>
        {(field) => (
          <div>
            <input {...field.props} value={field.input} type="password" />
            {field.errors && <div>{field.errors[0]}</div>}
          </div>
        )}
      </Field>
      <button type="submit" disabled={loginForm.isSubmitting}>
        {loginForm.isSubmitting ? "Submitting..." : "Login"}
      </button>
    </Form>
  );
}

Vue Example

<script setup lang="ts">
import { Field, Form, useForm } from "@formisch/vue";
import type { SubmitHandler } from "@formisch/vue";
import * as v from "valibot";

const LoginSchema = v.object({
  email: v.pipe(v.string(), v.email()),
  password: v.pipe(v.string(), v.minLength(8)),
});

const loginForm = useForm({
  schema: LoginSchema,
});

const handleSubmit: SubmitHandler<typeof LoginSchema> = (output) => {
  console.log(output);
};
</script>

<template>
  <Form :of="loginForm" @submit="handleSubmit">
    <Field :of="loginForm" :path="['email']" v-slot="field">
      <div>
        <input v-bind="field.props" v-model="field.input" type="email" />
        <div v-if="field.errors">{{ field.errors[0] }}</div>
      </div>
    </Field>
    <Field :of="loginForm" :path="['password']" v-slot="field">
      <div>
        <input v-bind="field.props" v-model="field.input" type="password" />
        <div v-if="field.errors">{{ field.errors[0] }}</div>
      </div>
    </Field>
    <button type="submit">Login</button>
  </Form>
</template>

SolidJS Example

import { Field, Form, createForm } from "@formisch/solid";
import type { SubmitHandler } from "@formisch/solid";
import * as v from "valibot";

const LoginSchema = v.object({
  email: v.pipe(v.string(), v.email()),
  password: v.pipe(v.string(), v.minLength(8)),
});

export default function LoginPage() {
  const loginForm = createForm({
    schema: LoginSchema,
  });

  const handleSubmit: SubmitHandler<typeof LoginSchema> = (output) => {
    console.log(output);
  };

  return (
    <Form of={loginForm} onSubmit={handleSubmit}>
      <Field of={loginForm} path={["email"]}>
        {(field) => (
          <div>
            <input {...field.props} value={field.input} type="email" />
            {field.errors && <div>{field.errors[0]}</div>}
          </div>
        )}
      </Field>
      <Field of={loginForm} path={["password"]}>
        {(field) => (
          <div>
            <input {...field.props} value={field.input} type="password" />
            {field.errors && <div>{field.errors[0]}</div>}
          </div>
        )}
      </Field>
      <button type="submit">Login</button>
    </Form>
  );
}

Svelte Example

<script lang="ts">
  import { createForm, Field, Form } from '@formisch/svelte';
  import type { SubmitHandler } from '@formisch/svelte';
  import * as v from 'valibot';

  const LoginSchema = v.object({
    email: v.pipe(v.string(), v.email()),
    password: v.pipe(v.string(), v.minLength(8)),
  });

  const loginForm = createForm({
    schema: LoginSchema,
  });

  const handleSubmit: SubmitHandler<typeof LoginSchema> = (output) => {
    console.log(output);
  };
</script>

<Form of={loginForm} onsubmit={handleSubmit}>
  <Field of={loginForm} path={['email']}>
    {#snippet children(field)}
      <div>
        <input {...field.props} value={field.input} type="email" />
        {#if field.errors}
          <div>{field.errors[0]}</div>
        {/if}
      </div>
    {/snippet}
  </Field>
  <Field of={loginForm} path={['password']}>
    {#snippet children(field)}
      <div>
        <input {...field.props} value={field.input} type="password" />
        {#if field.errors}
          <div>{field.errors[0]}</div>
        {/if}
      </div>
    {/snippet}
  </Field>
  <button type="submit">Login</button>
</Form>

Qwik Example

import { Field, Form, useForm$ } from "@formisch/qwik";
import { component$ } from "@qwik.dev/core";
import * as v from "valibot";

const LoginSchema = v.object({
  email: v.pipe(v.string(), v.email()),
  password: v.pipe(v.string(), v.minLength(8)),
});

export default component$(() => {
  const loginForm = useForm$({
    schema: LoginSchema,
  });

  return (
    <Form of={loginForm} onSubmit$={(output) => console.log(output)}>
      <Field
        of={loginForm}
        path={["email"]}
        render$={(field) => (
          <div>
            <input {...field.props} value={field.input.value} type="email" />
            {field.errors.value && <div>{field.errors.value[0]}</div>}
          </div>
        )}
      />
      <Field
        of={loginForm}
        path={["password"]}
        render$={(field) => (
          <div>
            <input {...field.props} value={field.input.value} type="password" />
            {field.errors.value && <div>{field.errors.value[0]}</div>}
          </div>
        )}
      />
      <button type="submit">Login</button>
    </Form>
  );
});

Form Configuration

const form = useForm({
  // Required: Valibot schema
  schema: MySchema,

  // Optional: Initial values (partial allowed)
  initialInput: {
    email: "user@example.com",
  },

  // Optional: When first validation occurs
  // Options: 'initial' | 'blur' | 'input' | 'submit' (default)
  validate: "submit",

  // Optional: When revalidation occurs after first validation
  // Options: 'blur' | 'input' (default) | 'submit'
  revalidate: "input",
});

Field Paths

Paths are type-safe arrays that reference fields in your schema.

// Top-level field
<Field of={form} path={['email']} />

// Nested field (schema: { user: { email: string } })
<Field of={form} path={['user', 'email']} />

// Array item field (schema: { todos: [{ label: string }] })
<Field of={form} path={['todos', 0, 'label']} />

// Dynamic array index
{items.map((item, index) => (
  <Field of={form} path={['todos', index, 'label']} key={item} />
))}

Form Methods

All methods follow a consistent API pattern:

• First parameter: Form store
• Second parameter: Config object

Reading Values

import { getInput, getErrors, getAllErrors } from "@formisch/react";

// Get field value
const email = getInput(form, { path: ["email"] });

// Get entire form input
const allInputs = getInput(form);

// Get field errors
const emailErrors = getErrors(form, { path: ["email"] });

// Get all errors across all fields
const allErrors = getAllErrors(form);

Setting Values

import { setInput, setErrors, reset } from "@formisch/react";

// Set field value (updates current input, not initial)
setInput(form, { path: ["email"], input: "new@example.com" });

// Set field errors manually
setErrors(form, { path: ["email"], errors: ["Email already taken"] });

// Clear errors
setErrors(form, { path: ["email"], errors: null });

// Reset entire form
reset(form);

// Reset with new initial values
reset(form, {
  initialInput: { email: "", password: "" },
});

// Reset but keep current input
reset(form, {
  initialInput: newServerData,
  keepInput: true,
});

Form Control

import { validate, focus, submit, handleSubmit } from "@formisch/react";

// Validate form manually
const isValid = await validate(form);

// Validate and focus first error field
await validate(form, { shouldFocus: true });

// Focus a specific field
focus(form, { path: ["email"] });

// Programmatically submit form
submit(form);

// Create submit handler for external buttons
const onExternalSubmit = handleSubmit(form, (output) => {
  console.log(output);
});

Field Arrays

For dynamic lists of fields, use FieldArray with array manipulation methods.

Schema

const TodoSchema = v.object({
  heading: v.pipe(v.string(), v.nonEmpty()),
  todos: v.pipe(
    v.array(
      v.object({
        label: v.pipe(v.string(), v.nonEmpty()),
        deadline: v.pipe(v.string(), v.nonEmpty()),
      }),
    ),
    v.nonEmpty(),
    v.maxLength(10),
  ),
});

React Example

import {
  Field,
  FieldArray,
  Form,
  useForm,
  insert,
  remove,
  move,
  swap,
} from "@formisch/react";

export default function TodoPage() {
  const todoForm = useForm({
    schema: TodoSchema,
    initialInput: {
      heading: "",
      todos: [{ label: "", deadline: "" }],
    },
  });

  return (
    <Form of={todoForm} onSubmit={(output) => console.log(output)}>
      <Field of={todoForm} path={["heading"]}>
        {(field) => <input {...field.props} value={field.input} type="text" />}
      </Field>

      <FieldArray of={todoForm} path={["todos"]}>
        {(fieldArray) => (
          <div>
            {fieldArray.items.map((item, index) => (
              <div key={item}>
                <Field of={todoForm} path={["todos", index, "label"]}>
                  {(field) => (
                    <input {...field.props} value={field.input} type="text" />
                  )}
                </Field>
                <Field of={todoForm} path={["todos", index, "deadline"]}>
                  {(field) => (
                    <input {...field.props} value={field.input} type="date" />
                  )}
                </Field>
                <button
                  type="button"
                  onClick={() =>
                    remove(todoForm, { path: ["todos"], at: index })
                  }
                >
                  Delete
                </button>
              </div>
            ))}
            {fieldArray.errors && <div>{fieldArray.errors[0]}</div>}
          </div>
        )}
      </FieldArray>

      <button
        type="button"
        onClick={() =>
          insert(todoForm, {
            path: ["todos"],
            initialInput: { label: "", deadline: "" },
          })
        }
      >
        Add Todo
      </button>

      <button type="submit">Submit</button>
    </Form>
  );
}

Array Methods

import { insert, remove, move, swap, replace } from "@formisch/react";

// Add item at end
insert(form, { path: ["todos"], initialInput: { label: "", deadline: "" } });

// Add item at specific index
insert(form, {
  path: ["todos"],
  at: 0,
  initialInput: { label: "", deadline: "" },
});

// Remove item at index
remove(form, { path: ["todos"], at: index });

// Move item from one index to another
move(form, { path: ["todos"], from: 0, to: 3 });

// Swap two items
swap(form, { path: ["todos"], at: 0, and: 1 });

// Replace item at index
replace(form, {
  path: ["todos"],
  at: 0,
  initialInput: { label: "New task", deadline: "2024-12-31" },
});

TypeScript Integration

Type Inference

Types are automatically inferred from your Valibot schema:

const LoginSchema = v.object({
  email: v.pipe(v.string(), v.email()),
  password: v.pipe(v.string(), v.minLength(8)),
});

const form = useForm({ schema: LoginSchema });
// form is FormStore<typeof LoginSchema>

// Submit handler receives typed output
const handleSubmit: SubmitHandler<typeof LoginSchema> = (output) => {
  output.email; // ✓ string
  output.password; // ✓ string
  output.username; // ✗ TypeScript error
};

Input vs Output Types

Schemas with transformations have different input and output types:

const ProfileSchema = v.object({
  age: v.pipe(
    v.string(), // Input: string
    v.transform((input) => Number(input)), // Output: number
    v.number(),
  ),
  birthDate: v.pipe(
    v.string(), // Input: string
    v.transform((input) => new Date(input)), // Output: Date
    v.date(),
  ),
});

// In Field: field.input is string
// In onSubmit: output.age is number, output.birthDate is Date

Type-Safe Props

Pass forms to child components with proper typing:

import type { FormStore } from "@formisch/react";

type FormContentProps = {
  of: FormStore<typeof LoginSchema>;
};

function FormContent({ of }: FormContentProps) {
  return (
    <Form of={of} onSubmit={(output) => console.log(output)}>
      {/* ... */}
    </Form>
  );
}

Generic Field Components

Create reusable field components with proper typing:

import { useField, type FormStore } from "@formisch/react";
import * as v from "valibot";

type EmailInputProps = {
  of: FormStore<v.GenericSchema<{ email: string }>>;
};

function EmailInput({ of }: EmailInputProps) {
  const field = useField(of, { path: ["email"] });

  return (
    <div>
      <input {...field.props} value={field.input} type="email" />
      {field.errors && <div>{field.errors[0]}</div>}
    </div>
  );
}

Available Types

import type {
  FormStore, // Form store type
  FieldStore, // Field store type
  FieldArrayStore, // Field array store type
  SubmitHandler, // Submit handler function type
  ValidPath, // Valid field path type
  ValidArrayPath, // Valid array field path type
  Schema, // Base schema type from Valibot
} from "@formisch/react";

Validation Timing

validate Option

Controls when the first validation occurs:

revalidate Option

Controls when validation runs after the first validation:

Special Inputs

Select (Single)

<Field of={form} path={["framework"]}>
  {(field) => (
    <select {...field.props}>
      {options.map(({ label, value }) => (
        <option key={value} value={value} selected={field.input === value}>
          {label}
        </option>
      ))}
    </select>
  )}
</Field>

Select (Multiple)

<Field of={form} path={["frameworks"]}>
  {(field) => (
    <select {...field.props} multiple>
      {options.map(({ label, value }) => (
        <option
          key={value}
          value={value}
          selected={field.input?.includes(value)}
        >
          {label}
        </option>
      ))}
    </select>
  )}
</Field>

Checkbox

<Field of={form} path={["acceptTerms"]}>
  {(field) => <input {...field.props} type="checkbox" checked={field.input} />}
</Field>

File Input

File inputs cannot be controlled. Handle via UI around them:

<Field of={form} path={["avatar"]}>
  {(field) => (
    <div>
      <input {...field.props} type="file" />
      {field.input && <span>{field.input.name}</span>}
    </div>
  )}
</Field>

useField Hook

For complex field components, use the useField hook instead of the Field component:

import { useField } from "@formisch/react";

function EmailInput({ form }) {
  const field = useField(form, { path: ["email"] });

  // Access field state in component logic
  useEffect(() => {
    if (field.errors) {
      console.log("Email has errors:", field.errors);
    }
  }, [field.errors]);

  return (
    <div>
      <input {...field.props} value={field.input} type="email" />
      {field.errors && <div>{field.errors[0]}</div>}
    </div>
  );
}

When to use which:

• Field component — Multiple fields in the same component
• useField hook — Single field with component logic access

Using Component Libraries

When using component libraries that don't expose their underlying native HTML elements, you cannot spread field.props directly. Instead, use field.onChange (React) or field.onInput (other frameworks) to update the value programmatically:

import { DatePicker } from "some-component-library";

<Field of={form} path={["date"]}>
  {(field) => (
    <DatePicker
      value={field.input}
      onChange={(newDate) => field.onChange(newDate)}
    />
  )}
</Field>;

The field.onChange method updates the field value and triggers validation, just like a native input would.

This is useful for:

• Component libraries that wrap native elements without exposing them
• Complex custom inputs like date pickers, rich text editors, or color pickers

Async Submission

const handleSubmit: SubmitHandler<typeof LoginSchema> = async (values) => {
  try {
    const response = await fetch("/api/login", {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify(values),
    });

    if (!response.ok) {
      // Set server-side errors
      const data = await response.json();
      setErrors(form, { path: ["email"], errors: [data.error] });
    }
  } catch (error) {
    console.error("Submission failed:", error);
  }
};

Common Patterns

Loading State

<button type="submit" disabled={form.isSubmitting}>
  {form.isSubmitting ? "Submitting..." : "Submit"}
</button>

Submit on Enter

Formisch handles this automatically via the native <form> element.

Reset After Success

const handleSubmit: SubmitHandler<typeof Schema> = async (values) => {
  await saveData(values);

  // Full reset to initial state
  reset(form);

  // Or reset but keep current input values
  reset(form, { keepInput: true });
};

Server Data Sync

When server data changes, update the baseline without losing user edits:

// After refetching data from server
reset(form, {
  initialInput: newServerData,
  keepInput: true, // Keep user's current edits
  keepTouched: true, // Keep touched state (optional)
});

Conditional Fields

<Field of={form} path={["hasAccount"]}>
  {(field) => <input {...field.props} type="checkbox" checked={field.input} />}
</Field>;

{
  getInput(form, { path: ["hasAccount"] }) && (
    <Field of={form} path={["accountId"]}>
      {(field) => <input {...field.props} value={field.input} />}
    </Field>
  );
}

Additional Resources

• Formisch Documentation
• Formisch GitHub
• Valibot Documentation