Why does a for loop with a semicolon after it still execute?

Question

I was having a look at some code a friend did and stumbled across this line which I thought was an error (simplified for example)..

for (g = 0; 10 > g; g+         


        

Answer 1

Your g variable is within the global context, so it is accessible outside of a for loop. The loop did its job, and incremented g 10 times.
Two {} indicate a block in JavaScript and won't cause any error.

Edit: it is not alerting anything without a for loop, because the g variable is not defined.