[javascript] How to set a value to a file input in HTML?

How can I set the value of this?

<input type="file" />

As everyone else here has stated: You cannot upload just any file automatically with JavaScript.

HOWEVER! If you have access to the information you want to send in your code (i.e., not C:\passwords.txt), then you can upload it as a blob-type, and then treat it as a file.

What the server will end up seeing will be indistinguishable from someone actually setting the value of <input type="file" />. The trick, ultimately, is to begin a new XMLHttpRequest() with the server...

function uploadFile (data) {
        // define data and connections
    var blob = new Blob([JSON.stringify(data)]);
    var url = URL.createObjectURL(blob);
    var xhr = new XMLHttpRequest();
    xhr.open('POST', 'myForm.php', true);
    
        // define new form
    var formData = new FormData();
    formData.append('someUploadIdentifier', blob, 'someFileName.json');
        
        // action after uploading happens
    xhr.onload = function(e) {
        console.log("File uploading completed!");
    };
    
        // do the uploading
    console.log("File uploading started!");
    xhr.send(formData);
}

    // This data/text below is local to the JS script, so we are allowed to send it!
uploadFile({'hello!':'how are you?'});

So, what could you possibly use this for? I use it for uploading HTML5 canvas elements as jpg's. This saves the user the trouble of having to open a file input element, only to select the local, cached image that they just resized, modified, etc.. But it should work for any file type.

You can't. And it's a security measure. Imagine if someone writes JS that sets file input value to some sensitive data file?

Actually we can do it. we can set the file value default by using webbrowser control in c# using FormToMultipartPostData Library.We have to download and include this Library in our project. Webbrowser enables the user to navigate Web pages inside form. Once the web page loaded , the script inside the webBrowser1_DocumentCompleted will be executed. So,

private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
    {
       FormToMultipartPostData postData = 
            new FormToMultipartPostData(webBrowser1, form);
        postData.SetFile("fileField", @"C:\windows\win.ini");
        postData.Submit();
    }

Refer the below link for downloading and complete reference.

https://www.codeproject.com/Articles/28917/Setting-a-file-to-upload-inside-the-WebBrowser-com

Not an answer to your question (which others have answered), but if you want to have some edit functionality of an uploaded file field, what you probably want to do is:

• show the current value of this field by just printing the filename or URL, a clickable link to download it, or if it's an image: just show it, possibly as thumbnail
• the <input> tag to upload a new file
• a checkbox that, when checked, deletes the currently uploaded file. note that there's no way to upload an 'empty' file, so you need something like this to clear out the field's value

Define in html:

<input type="hidden" name="image" id="image"/>

In JS:

ajax.jsonRpc("/consulta/dni", 'call', {'document_number': document_number})
    .then(function (data) {
        if (data.error){
            ...;
        }
        else {
            $('#image').val(data.image);
        }
    })

After:

<input type="hidden" name="image" id="image" value="/9j/4AAQSkZJRgABAgAAAQABAAD/2wBDAAgGBgcGBQgHBwcJCQgKDBQNDAsLDBkSEw8U..."/>
<button type="submit">Submit</button>

You can't.

The only way to set the value of a file input is by the user to select a file.

This is done for security reasons. Otherwise you would be able to create a JavaScript that automatically uploads a specific file from the client's computer.