这是程序：

var height = prompt("How high do you 
want the grid? (1-10 is good)","10");

var width= prompt("How wide do you want 
the grid? (1-10 is good)","10");

var a_line;

var new_window = 
window.open("/webmonkey/98/04/files1a
/grid.html","looper","width=400,height=400");

new_window.document.writeln("<h1>A Grid</h1>");

for (height_loop = 0; height_loop 
< height; height_loop++) 

{	

	a_line = "";

	for (width_loop = 0; 
width_loop < width; width_loop++)

	{

		a_line+="x";

	}

	new_window.document.writeln
(a_line + "<br>");

}

在请求高及宽后，打开一新窗口，并为它写一个头，进入for循环。第一个for循环设a_line=“”。试一下不用该行做一下本例，看看会发生什么。在初始化a_line后，程序进入下一个for循环，当宽达到所需值后，建立X`S行并将它显现在新窗口中。这些将发生height次。

好，现在你的任务：看一下这个循环练习并亲自试着在看源码前写一个。