[whatwg/xhr] FormData: Add ability to specify submitter in addition to <form> (#262)

Context: https://github.com/whatwg/html/issues/3195
Context: https://github.com/whatwg/xhr/issues/202

FormData should have ability to add an entry for a submitter button when it appends entries for a <form>.

We have multiple options of how to specify submitter.  See five comments since https://github.com/whatwg/html/issues/3195#issuecomment-540927844

----

A) `constructor(optional (HTMLFormElement or record<USVString, FormDataEntryValue>) formOrMap, optional HTMLElement? submitter = null)`
Idiomatic.

B) `constructor(optional (HTMLElement or record<...>) formOrSubmitterOrMap)`
`HTMLElement` represents a form or a submitter.

C) `constructor(optional (HTMLFormElement or FormDataInit) formOrDict)`
```
dictionary FormDataInit {
 HTMLFormElement form;
 HTMLElement? submitter;
 record<...> map;
}
```
The content of the dictionary is idiomatic.
Need to wrap a record with a dictionary.
Extensible. It's easy to add new members to the dictionary in the future.

D) `constructor(optional (HTMLFormElement or FormDataInit or URLSearchParams) init)`
```
dictionary FormDataInit {
  required HTMLFormElement form;
  HTMLElement? submitter = null;
}
```
`FormData` doesn't support `record<>` directly. Developers have to write `new FormData(new URLSearchParams(map))`.

E) `constructor(optional HTMLFormElement form, optional HTMLElement? submitter = null)`
`FormData append(record<USVString, FormDataEntryValue> map);`
Developers have to write `new FormData().append(map)`.

F) (no changes on the constructor)
```
FormData append(HTMLFormElement form, optional HTMLElement? submitter = null);
FormData append(record<USVString, FormDataEntryValue> map);
```
Developers have to write `let fd = new FormData().append(form, submitter);` if they want to collect entries including an entry for the submitter.


-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/262