tanstack-form

name: tanstack-form description: > Use this skill when building or modifying forms with TanStack Form and Zod validation. Covers createForm, field-level validation, error handling, and mapping ProblemDetails API errors to form fields. Apply when adding new forms, implementing validation logic, or handling form submission in the Svelte frontend.

TanStack Form

Documentation: tanstack.com/form. Use official docs when the local pattern is not enough.

Use TanStack Form (@tanstack/svelte-form) with Zod for form state management.

Zod Schema Generation

Schemas are generated from backend models and extended in feature slices:

// Generated in $generated/schemas.ts (auto-generated from backend)
export const LoginSchema = object({ email: email(), password: string() });
export type LoginFormData = Infer<typeof LoginSchema>;

// Extended in feature schemas.ts
// From src/lib/features/auth/schemas.ts
import { ChangePasswordModelSchema } from "$generated/schemas";

export const ChangePasswordSchema = ChangePasswordModelSchema.extend({
    confirm_password: string().min(6).max(100),
}).refine((data) => data.password === data.confirm_password, {
    message: "Passwords do not match",
    path: ["confirm_password"],
});
export type ChangePasswordFormData = Infer<typeof ChangePasswordSchema>;

// Re-export generated schemas
export { LoginSchema, type LoginFormData } from "$generated/schemas";

Basic Form Pattern

From src/Exceptionless.Web/ClientApp/src/routes/(auth)/login/+page.svelte:

<script lang="ts">
    import { createForm } from '@tanstack/svelte-form';
    import * as Field from '$comp/ui/field';
    import { Input } from '$comp/ui/input';
    import { Button } from '$comp/ui/button';
    import { type LoginFormData, LoginSchema } from '$features/auth/schemas';
    import { ariaInvalid, mapFieldErrors, problemDetailsToFormErrors } from '$shared/validation';

    const form = createForm(() => ({
        defaultValues: {
            email: '',
            password: ''
        } as LoginFormData,
        validators: {
            onSubmit: LoginSchema,
            onSubmitAsync: async ({ value }) => {
                const response = await login(value.email, value.password);
                if (response.ok) {
                    await goto('/');
                    return null;
                }
                return problemDetailsToFormErrors(response.problem);
            }
        }
    }));
</script>

<form onsubmit={(e) => { e.preventDefault(); form.handleSubmit(); }}>
    <form.Field name="email">
        {#snippet children(field)}
            <Field.Field data-invalid={ariaInvalid(field)}>
                <Field.Label for={field.name}>Email</Field.Label>
                <Input
                    id={field.name}
                    type="email"
                    value={field.state.value}
                    onblur={field.handleBlur}
                    oninput={(e) => field.handleChange(e.currentTarget.value)}
                    aria-invalid={ariaInvalid(field)}
                />
                <Field.Error errors={mapFieldErrors(field.state.meta.errors)} />
            </Field.Field>
        {/snippet}
    </form.Field>

    <form.Subscribe selector={(state) => state.isSubmitting}>
        {#snippet children(isSubmitting)}
            <Button type="submit" disabled={isSubmitting}>
                {isSubmitting ? 'Logging in...' : 'Log In'}
            </Button>
        {/snippet}
    </form.Subscribe>
</form>

Server Error Handling

Convert ProblemDetails to form errors:

onSubmitAsync: async ({ value }) => {
    const response = await login(value.email, value.password);
    if (response.ok) return null;
    return problemDetailsToFormErrors(response.problem);
};

Form in Dialog Pattern

Close dialog only after successful submission:

let open = $state(false);

const form = createForm(() => ({
    defaultValues: { name: '' },
    validators: {
        onSubmit: mySchema,
        onSubmitAsync: async ({ value }) => {
            try {
                await createMutation.mutateAsync(value);
                open = false;
                return null;
            } catch (error: unknown) {
                if (error instanceof ProblemDetails) {
                    return problemDetailsToFormErrors(error);
                }
                return { form: 'An unexpected error occurred' };
            }
        }
    }
}));

References

See shadcn-svelte for Field component patterns.