What does a Ajax call response like ‘for (;;); { json data }’ mean? [duplicate]

Possible Duplicate:
Why do people put code like “throw 1; <dont be evil>” and “for(;;);” in front of json responses?

I found this kind of syntax being used on Facebook for Ajax calls. I’m confused on the for (;;); part in the beginning of response. What is it used for?

This is the call and response:

GET http://0.131.channel.facebook.com/x/1476579705/51033089/false/p_1524926084=0

Response:

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