What is the point to put deadlock code ahead of JSON in HTTP response?

Question

When sniffing Gmail and Facebook traffic, I found there are leading deadlock code before JSON response of XmlHttpRequest.

For example

for (;;);{"t":"continue"}

and

while(1); [["v","nW3OxUDq0kU.en.","8","51bec53f21305d9c"],["di",86]]

What is the purpose of this "for(;;);" and "while(1);" deadlock?

Answer 1

It's to prevent people from remotely getting to that data from a remote domain by creating a <script> to request it. Of course it has no effect on an XMLHttpRequest because with it you can just skip the infinite loop.