var a_line="";
var loop = 0;
while (loop < width)
{
	a_line = a_line + "x";
	loop=loop+1;
}

... could have been written using a for loop, like this:

var a_line="";
for (var loop=0; loop < width; loop++)
{
	a_line = a_line + "x";
}

for loops come in this form:

for (initial value; test; increment) 
{
	do this stuff;
}

So, the above for loop sets loop = 0 and says that you should keep adding 1 to loop as long as loop < width. It's just like the previous while loop with a few less lines. Both say "add an x to a_line width times."

One more thing about loops before we do an exercise using them: Loops can be nested. Here's an example of nested loops.