Why is FormData null value converted to “null” when send to Laravel?

I have this sample JavaScript:

let data = new FormData();
data.set('mode_id', '');
data.set('start', null);
axios({
    method: 'POST',
    url: '/test',
    data: data,
    headers: {
      'Content-Type': 'multipart/form-data'
    }
})

This is the controller endpoint in Laravel:

public function store(Request $request)
{
    Debugbar::info($request->all());

    $request->validate([
        'start'        =>  'required',
        'mode_id'      =>  'required',
    ]);

To my surprise, the validation for parameter mode_id failed, but not for parameter start.

From Debugbar, I find that start value is a string "null".

[
  "mode_id" => null
  "start" => "null"
]

Why and where is FormData null converted to “null”?

If I do a PHPUNIT test

$response = $this->->post('/test', [
        'mode_id' => '',
        'start' => null,
 ]);

Both parameter mode_id and start fail in validation, as expected.

Answers:

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Method 1

Okay, so I found it here https://developer.mozilla.org/en-US/docs/Web/API/FormData/set:

formData.set(name, value);

Parameters

name

The name of the field whose data is contained in value.

value

The field's value. This can be a USVString or Blob (including subclasses
such as File). 
If none of these are specified the value is converted to a string.

In other words, one cannot set null as a value with FormData. I have to catch on the FrontEnd a null value, replace it by an empty string and Laravels Middleware IlluminateFoundationHttpMiddlewareConvertEmptyStringsToNull will convert this back to null.

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