First, we ask for the number of x's:

var width = prompt("How many x's would you like?
(1-10 is good)","5");

Next, we declare a few variables:

var a_line="";
var loop = 0;

And now for the important part:

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

This says, "while the variable loop is less than the requested width of the row of x's, add another x to the line and then add one to the value of loop." This loop will keep adding an x to the line and adding one to the value of loop until loop is no longer less than the requested width. Here's a timeline of what happens when a person chooses two x's at the prompt:

Time 1

• a_line = "" (because we initialized it to be "")
• loop=0 (because we initialized it to be 0)
• width=2 (because that's what the user asked for)
• 0 is less than 2 so
• a_line = a_line + "x", so now a_line = "x"
• loop=loop+1, so now loop = 1

Back into the loop: Time 2

• loop=1
• width=2
• a_line = "x"
• 1 is less than 2 so
• a_line = a_line + "x", so now a_line = "xx"
• loop=loop+1, so now loop = 2

Back into the loop: Time 3

• loop=2
• width=2
• a_line = "xx"
• 2 is NOT less than 2 so
• fall out of the loop and do what follows

alert(a_line);

Throw up an alert box announcing a_line.

This sort of loop is so common that programmers have developed a few shortcuts. Using the shortcuts, the while loop could have been written like this:

while (loop < width)
{
  a_line += "x"; //this was a_line = a_line + "x";
  loop++;        //this was loop=loop+1;
}

The first line, a_line += "x", says "add x to myself." This shortcut works with numbers, too. If you have a_number = 5, and you write, a_number+=3, it's just like writing a_number=a_number+3. Programmers are lazy; they're always coming up with shortcuts like this.

The next line, loop++, means "add one to myself." So, loop++ is the same as loop=loop+1, which could also be written loop+=1. Each of these is equally good. Which one you use depends on how lazy you are.

Just like there's more than one way to add 1 to a number, there's more than one way to write a loop. while loops aren't the only kind of loops out there. Another popular one is the for loop.