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

Modifying File Objects and FileList Length within a FormData Object

Question:

How can we modify the File objects within a FileList and set its length, ensuring the files are reflected in the FormData object?

Answer:

Using DataTransfer

As discovered by the original poster (OP), the DataTransfer API offers a method to accomplish this task, despite its current limitations (supported only by Blink, FF >= 62). By creating a mutable FileList through the DataTransferItemList, we can set arbitrary files on the FileList of an HTML file input element.

// Create a new DataTransfer object<br>const dT = new DataTransfer();</p><p>// Add a File object to the DataTransferItemList<br>dT.items.add(new File(['foo'], 'programmatically_created.txt'));</p><p>// Set the files property of the input element to the DataTransfer object's files<br>inp.files = dT.files;<br>

<input type="file">

This approach effectively modifies the FileList within the file input element, and the updated files will be reflected in the FormData object.

Limitations:

It's important to note that this technique currently has limited browser support. Additionally, some fallback behavior may be required for browsers that do not support the DataTransfer constructor.

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