How to run jQuery directly on any page in the browser?

Question

Is there some programmatic way (or maybe a browser plugin) that allows users to arbitrarily run any jQuery they want on a webpage that is currently loaded in their browser?

Answer 1

eval executes any string you pass it, but if its usage is 'proper' is contested.

eval('$(function() { alert("hello"); }');

will show an alert on document ready (provided that jQuery is sourced before the eval is called).