How come the program outputs 123?

Question

var i=1;
do {
document.write(i + "<br />");
i++;
}
while(Math.floor(i/2) == 1);

Answer 1

Well, here we have a do while loop (works at least once) and a floor function that rounds the result of division down. Do statement executes before evaluating the while condition.
Step 1 var is 1, it gets printed out and increased by 1.
Do statement is done.
Step 2 while var is 2 it is divided by 2 and the result equals 1. Hence while is true the loop continues.
Step 3 var equals 3.
3 divided by 2 equals 1.5 but it is still 1 rounded down. The result gets printed out and i becomes 4.
Finally 4 divided by 2 is 2.
The loop breaks.

Answer 2

Memorize these three words:
Run. Through. It.
i is 1. It's printed, then incremented. 2 / 2 is 1, then i is printed; then incremented again, to 3, 3 / 2 is 1.5, and 1.5's floor is 1. i is printed as 3, then incremented. At this point i is 4, 4 / 2 is 2, and that's no good for the loop :[
TL;DR
print (1)
i = 2 -> 2 / 2 = 1
print (2)
i = 3 -> 3 / 2 = 1(.5)
print (3)
i = 4 -> 4 / 2 = 2 -> break