How Can I Modify File Objects and FileList Length within FormData?

Editing File Objects and FileList Properties in FormData

Question:

How can we edit File objects and adjust the length property of the FileList within a FormData object?

Background:

FileList objects typically have a Symbol.iterator property, allowing us to set objects that are iterable as File types. However, the files .length property remains at 0.

Solution:

A recent breakthrough has emerged, as demonstrated by the OP in their gist. Utilizing the DataTransfer constructor, we can create a mutable FileList accessible via DataTransferItemList.

Details:

In Blink browsers and Firefox versions 62 and above, the DataTransfer constructor can create a mutable FileList. Prior to Firefox 62, a bug in the implementation of ClipboardEvent provided a workaround.

Code Implementation:

Here's an example of how to achieve this:

const dT = new DataTransfer();
dT.items.add(new File(['foo'], 'programmatically_created.txt'));
inp.files = dT.files;

<input type="file">

This approach effectively modifies the file objects and sets the length property of the FileList correctly, allowing them to be reflected in the FormData object.

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