Beautify and format minified JavaScript online

No. Minifiers rename variables to short identifiers and that mapping is not stored in the output. If a source map was published alongside the file, that is what restores names.

Formatting alone does not. The one thing to watch is that automatic semicolon insertion means line breaks can matter, which is why relying on ASI in the first place is risky.

Yes, including arrow functions, classes, optional chaining and template literals. JSX is not JavaScript, so a JSX file will format oddly.

Yes, that is the common use. Bear in mind that reading a script tells you what it does today, and a hosted script can change at any time.

No. The formatter is loaded into the page and runs there, so proprietary code stays on your machine.