Attach a file to an input with javascript

Question

I have an input to upload files

<input type="file" name="comment[video_file]" id="comment_video_file">

Is it possible to attach a file by JavaScript?

I have tried but it didn't work

    let file = new File([videoBlob], "video.mp4", { type: "video/mp4" });
    let element = document.getElementById("comment_video_file");
    element.append("video", video);

If I console log the element after the append it looks like this

<input type="file" name="comment[video_file]" id="comment_video_file">
  "video"
  "[object File]"
</input>

The files property on an input is a read only property, as far as I am aware. — Taplar
– Taplar, Commented Jul 30, 2020 at 21:40
You can't do this, the input can only contain files selected directly by the user themselves, from the file system on their computer/device. If you need to upload this blob then you can do that with JavaScript, but not via this method — ADyson
– ADyson, Commented Jul 30, 2020 at 21:46
So, I can't attach it to the input or the form? How can I do it? — sara lance
– sara lance, Commented Jul 30, 2020 at 21:47
"So, I can't attach it to the input or the form? How can I do it?" As previously stated YOU CAN'T. It's a security restriction. The file(s) have to be selected by the user. — gforce301
– gforce301, Commented Jul 30, 2020 at 21:51
"How can I do it?" ...usually by sending the data via AJAX within a FormData object. A little bit of your own research would help you discover this, and discover some examples. — ADyson
– ADyson, Commented Jul 31, 2020 at 8:57

UI_Dev · Accepted Answer · 2020-07-30 21:59:22Z

3

It isn't possible to create a file and attach it to an HTML form input but using the FormData object you could send a generated file to the server as part of a post request.

Pulled from the MDN:

var formData = new FormData();

// JavaScript file-like object
var content = '<a id="a"><b id="b">hey!</b></a>'; // the body of the new file...
var blob = new Blob([content], { type: "text/xml"});

formData.append("webmasterfile", blob);

var request = new XMLHttpRequest();
request.open("POST", "http://example.com/submitform.php");
request.send(formData);

Which should get you the same result of a file generated by JS sent to the server.

answered Jul 30, 2020 at 21:59

UI_Dev

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Comments

kos · Accepted Answer · 2022-01-25 15:31:09Z

0

MDN article on file inputs states:

You can set as well as get the value of HTMLInputElement.files in all modern browsers; this was most recently added to Firefox, in version 57

I tried setting input.files to a files property from a drag-n-drop files event and it worked. Though I'm not sure if it's possible to do it with a manually created File.

If this won't work, try sending an XMLHttpRequest with FormData with a file Blob, as advised in other answers/comments.

answered Jan 25, 2022 at 15:31

kos

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