This program allows to pack JavaScript code into URL.

For example, alert('Hello, world!') can be packed into https://vort.github.io/u2j/#44TE3Sa5UsgimbFfeQ9Rj39w2sR

It allows to host small programs inside the link itself.
Unpacking is performed by static html page, data after # is not transmitted to the server.

Here is an example of simple games packed this way: Pac-Man (source), 2048 (source).