Javascript upload from local file to string? [duplicate]

I want to make an HTML form, with a Browse button to choose a file to upload, and a Submit button.

I know how to do that if I was making it submit the file to the server. But what if I want to parse the file in JS? Can I make the submitted file into a Blob or a string somehow?

This must be straightforward, but I’ve searched around and I can’t find the answer!