How to Implement File Uploads with JavaScript?

JavaScript File Upload

Uploading files using JavaScript requires a sequence of steps to capture the selected file, craft the request, and handle notifications.

Capturing the File

When a user clicks the file input button, the browser allows them to select a file via a "File open..." dialog. The selected file's name is stored in:

document.getElementById("image-file").value

Crafting the Request

To send the file to a server, we use a multi-part POST request. A common way to build such a request is using the FormData API:

let formData = new FormData();
formData.append("photo", photo);

Sending the Request

The fetch API allows for asynchronous file upload:

fetch('/upload/image', {method: "POST", body: formData});

Listening for Notifications

To listen for completion of the upload, several approaches can be used:

1. XHR Progress Events: Use the XMLHttpRequest object to track upload progress and completion.
2. File Reader with Custom Events: Read the file contents using FileReader and dispatch custom events for progress and completion.
3. Libraries: Leverage JavaScript libraries like Axios or Multer, which simplify file uploads and provide built-in progress handling capabilities.

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