`enhance` submit function `formData` re-assignable does nothing / no type error

[brunnerh]

Describe the bug

Via enhance, additional form data can be added, but reassigning the formData as a whole does nothing, nor does it cause a type error.

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-vylhwt?file=src%2Froutes%2F%2Bpage.svelte

Relevant example:

<script>
    import { enhance } from '$app/forms';

    function createData() {
        const data = new FormData();
        data.append('greeting', 'Hello World');

        return data;
    }
</script>

<form method="post" use:enhance={e => {
    e.formData = createData();

    // this would work:
    // for (const [key, value] of createData())
    //     e.formData.append(key, value);
}} >
    <button>Submit</button>
</form>

Logs

No response

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 18.18.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 9.4.2 - /usr/local/bin/npm
    pnpm: 8.10.5 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/adapter-auto: ^2.0.0 => 2.1.1 
    @sveltejs/kit: ^1.27.4 => 1.27.6 
    svelte: ^4.2.7 => 4.2.7 
    vite: ^4.4.2 => 4.5.0

Severity

annoyance

Additional Information

No response

[alvissraghnall]

WHY IS THERE NO RESPONSE TO THIS!? I'M CURRENTLY DEALING WITH THIS ISSUE AS WELL-

[brunnerh]

The workaround is simple: Modify the existing formData.