How to Programmatically Set and Update FileList Length for FormData in JavaScript?

Setting File Objects and Length in FileList for FormData Objects

Problem Statement:

How can we set File objects within a FileList object while also updating the length property of the FileList and ensuring that the changes are reflected in the corresponding FormData object?

Solution:

It is possible to set the .files property of a element to a FileList object, but the .files.length property initially remains set to 0. Additionally, when the form is submitted, the size property of the File object may be set to 0.

To address these issues, we can utilize the DataTransfer constructor. The DataTransfer object allows us to create a mutable FileList object that can be accessed through the DataTransferItemList. Once we have a mutable FileList object, we can set the File objects and update the length property accordingly.

Here's how you can implement this technique:

const input = document.createElement("input");
input.type = "file";
input.name = "files";
input.multiple = true;

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

input.files = dT.files;

This code snippet creates a new element and a DataTransfer object. It then adds a file to the DataTransfer object and sets the .files property of the input element to the FileList object from the DataTransfer object.

Now, when you access the .files property of the input element, you will have access to the File object(s) set through the DataTransfer object, and the .length property of the FileList object will be set correctly.

Furthermore, when the form is submitted, the File object(s) will have their size property set to the correct value. This approach ensures that the changes made to the FileList object are reflected in the FormData object that is submitted with the form.

The above is the detailed content of How to Programmatically Set and Update FileList Length for FormData in JavaScript?. For more information, please follow other related articles on the PHP Chinese website!