File API

Summary

Lets JavaScript read the contents of files a user selects or drags in — the basis of file upload previews, drag-and-drop uploads and client-side file parsing. File objects expose metadata and content for user-selected files, while Blob provides a general representation of immutable binary data.

• 1Build a file input that previews a selected image before upload. Keep the first version small, then add one improvement after the core behavior works.
• 2Use `FileReader` to read a text or CSV file's contents in the browser. Verify the behavior with realistic data and handle the failure or empty state as well.
• 3Learn `File`, `Blob` and `FileReader` — the three core pieces of this API.