Type instantiation is excessively deep and possibly infinite with componentized fields

[fzembow]

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Description

I'm trying to even further componentize the approach in the componentization guide, by allowing users to pass an array of the FormPathLeaves to render the form fields.

However, I'm seeing a TS error, "Type instantiation is excessively deep and possibly infinite" when trying to use FormPathLeaves as a variable, rather than hardcoding it. You can see three parallel approaches in the below snippet.

<script lang="ts">
  import { z } from 'zod';
  import type {
    SuperValidated,
    Infer,
    FormPathLeaves,
  } from 'sveltekit-superforms';
  import { superForm } from 'sveltekit-superforms';
  import TextField from './TextField.svelte';

  export const userFormSchema = z.object({
    name: z.string().default('Hello world!'),
    email: z.string().email(),
  });

  type UserFormSchema = typeof userFormSchema;

  export let data: SuperValidated<Infer<UserFormSchema>>;
  const pathLeaves: FormPathLeaves<Infer<UserFormSchema>>[] = ['name', 'email'];

  const constPathLeaves = ['name', 'email'] as const;

  const form = superForm(data);
  const { enhance } = form;
</script>

<form method="POST" use:enhance>
  {#each pathLeaves as field}
    <!-- TS error: Type instantiation is excessively deep and possibly infinite. -->
    <TextField superform={form} {field} />
  {/each}

  <!-- Directly specifying the values is fine. -->
  <TextField superform={form} field="name" />
  <TextField superform={form} field="email" />

  {#each constPathLeaves as field}
    <!-- Using "as const" is also fine. -->
    <TextField superform={form} {field} />
  {/each}

  <div><button>Submit</button></div>
</form>

This may very well be some limitation of my typescript knowledge... but is this just not intrinsically possible? Should I be listing out the paths with a different type annotation?

If applicable, a MRE

See this example, which has the error on Sveltelab when you view the LoginForm.svelte file: EXAMPLE

[ciscoheat]

I can't see any LoginForm.svelte in the example link?

[fzembow]

@ciscoheat I'm sorry - I guess sveltelab didn't save my changes earlier :(

Here is a corrected repro link: https://www.sveltelab.dev/fh9ym08604bfrxv

If you click into LoginForm.svelte now, the issue will appear

Again, this might be a limitation of Typescript (or my knowledge thereof) but I am very thankful that you would take a look -- to me, extending the componentization even further was the natural thing to do.

[ciscoheat]

I see now, I should have looked closer at the code example. You need const, otherwise it will treat each value as string and that will probably mess up the typing. The types behind the scenes are quite complicated, especially with generics.

I did manage to remove the error once locally, so try updating to latest typescript (and maybe latest svelte 5) and maybe it will work.

[macmillen]

I have the same problem

[fzembow]

FWIW I've just upgraded to svelte5 and all my other packages to latest and I can no longer reproduce the original issue.

[macmillen]

This still happens to me with Svelte 5:

radio-item.svelte

<script lang="ts" generics="T extends Record<string, unknown>">
  import type { FormPathLeaves, SuperForm } from "sveltekit-superforms";
  import RadioInput from "./radio-input.svelte";

  interface Props {
    superForm: SuperForm<T>;
    field: FormPathLeaves<T>;
    value: string | number;
  }

  let { superForm, field, value }: Props = $props();
</script>

<RadioInput {superForm} {field} {value}></RadioInput>

radio-input.svelte

<script lang="ts" generics="T extends Record<string, unknown>">
  import type { FormPathLeaves } from "sveltekit-superforms";
  import { fieldProxy, type SuperForm } from "sveltekit-superforms";

  interface Props {
    superForm: SuperForm<T>;
    field: FormPathLeaves<T>;
    value: string | number;
  }

  let { superForm, field, value }: Props = $props();

  const groupValue = fieldProxy(superForm, field);

  let id = $derived(`${value}-${field}`);
</script>

<input {id} type="radio" class="peer sr-only" name={field} {value} bind:group={$groupValue} />

[fzembow]

You're right, I'm still seeing this :(