Breaking this down a bit:
statements until the first ";" (semi-colon) is reached.
statements between the first ";" and the second ";".
follow the last ";".
A very simple FOR statement that outputs the numbers from
1 to 10:
for (var _i=1;_i<=10;_i++) document.write(_i+"<BR>");
This FOR loop declares a variable "_i" to be only valid
inside the for loop and preset to the value of 1.
The test for this loop to be finished is checked:
_i<=10 (_i is 1 and 1 is less than or equal to 10) so
the first itieration of the loop starts.
The document.write statement is the only statement inside
the loop since there are no curly braces for the body
of the FOR loop. I is displayed along with a BR tag (1<BR>)
After each iteration of the loop, the value _i is incremented by 1
(_i++). So the next evaluation for exiting the loop is tested:
_i<=10 (2<=10)... This continues until _i is 11 - at which point
of the for loop is processed.