Browser bookmarks can run little bits of javascript on the current page if you use a "javascript:" URL. Folks often refer to these as "bookmarklets". (I wish they worked well in mobile browsers like iOS Safari, but that's a rant...)

Constructing scripts in a URL-safe format can be super frustrating! But today I expanded my "dev tools" for building bookmarklets:

The Firefox dev tools console is a great way to try out your code in-place on the actual page where you want to run it. Press "Ctrl+b" (or "Cmd+b" if you are a Mac person) and the console editor goes into multiline mode! This makes it much easier to edit little scripts! In multiline mode, you "submit" by pressing "Ctrl+Enter" (or "Cmd+Enter" etc).

Duckduckgo has a built-in tool for Javascript beautifying and minifying! It pops up when you search duckduckgo for "javascript minifier". This makes it easy to go from a readable script with newlines and other formatting in your console, to a minimized compact scriptlet suitable for a bookmarklet. Even better, it lets you go back and forth between a minimized script and a readable one so you can tweak your bookmarklets in the future.

Finally, and in some ways most importantly, bookmarklets need to be properly URL-encoded. Enter Marek Gibney's Bookmarklet Editor, which uses a single textarea and lets you convert between a script and its encoded "javascript:..." version.

The minifier/beautifier and bookmarklet editor are web tools, so the usual caveats apply about data privacy. In theory they are running purely in your browser and not sending data anywhere. But they could! So keep that in your threat model.