The Javascript Trap by Richard M. Stallman
In the free software community, the idea that non-free programs mistreat their users is familiar. Some of us refuse entirely to install proprietary software, and many others consider non-freedom a strike against the program. Many users are aware that this issue applies to the plug-ins that browsers offer to install, since they can be free or non-free.
But browsers run other non-free programs which they don't ask you about or even tell you about—programs that web pages contain or link to. These programs are most often written in Javascript, though other languages are also used.