Make HTMLInputElement.prototype.files settable

[tkent-google]

https://www.w3.org/Bugs/Public/show_bug.cgi?id=22682

It would be useful if web authors can do:
input.files = dataTransfer.files;
to upload dropped files.

WebKit and Blink implemented this behavior. (Actually, it was implemented before Blink fork.)

[annevk]

Can you explain what exactly is implemented? Do you basically allow setting it to any array-like, throw for non-File, and always return a FileList? Do Blob objects get upgraded as they do with FormData?

[tkent-google]

Current implementation of WebKit and Blink:

For now, FileList is a normal interface, not an Array. The files setter accepts only a FileList object, and never accepts Blobs.
An HTMLInputElement object retains the passed FileList objet, and files getter returns it as is.

inputFile2.files = inputFile1.files;
inputFile2.files === inputFile1.files
>>> true
inputFile2.files[0] === inputFile1.files[0]
>>> true

I also found Edge's files was settable. In Edge, it seems the files setter does deep-copy. i.e.

inputFile2.files = inputFile1.files;
inputFile2.files === inputFile1.files
>>> false
inputFile2.files[0] === inputFile1.files[0]
>>> false

But Edge didn't accept a normal Array of Files.

[annevk]

How do you get FileList then? Can you construct one in WebKit and Blink?

[tkent-google]

How do you get FileList then? Can you construct one in WebKit and Blink?

We can get it only by files of another HTMLInputElement instance, or files of DataTranfer. There are no other ways AFAIK.

[annevk]

That makes the API a little bit useless, no? But I guess we can define that as an initial setup.

[annevk]

I wrote a PR and tests. Review appreciated.

[annevk]

I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1384030 by the way. @domenic if I file an issue against Edge is this good to go or is there something else you're waiting for? It's already implemented per proposed specification by two browsers after all.

[domenic]

Looks good, am just behind on reviews due to TC39. Had some test suggestions.

[guest271314]

Curious what the use cases are?

[annevk]

See OP, uploading dropped files.