Count, compare and clean up text.
Counting, comparing and cleaning up text. Simple jobs that are irritating to do by hand and trivial to do in a browser tab.
The counting and comparing happen as you type, in this tab — nothing you paste into those is sent anywhere, which is also why they stay fast on a document of any length.
Summarising and rewriting are the exceptions, because no amount of JavaScript does either. Those two send your text to a server and say so at the top of their pages before you paste anything in. If a document is sensitive enough that this matters, the distinction is on the page rather than in the small print.