js--小结⑤

js中的for循环，while循环，do...while循环和C语言的一模一样

有几个问题要提醒一下的是

1. null是对象，即object undefined是undefined

demo

 1 <!DOCTYPE html>
 2 <html>
 3 <body>
 4 
 5 <p id="demo"></p>
 6 
 7 <script>
 8 document.getElementById("demo").innerHTML =
 9 typeof undefined + "<br>" +
10 typeof null + "<br>" +
11 (null === undefined) + "<br>" +
12 (null == undefined);
13 </script>
14 
15 </body>
16 </html> 
17

结果：

2.几个demo

break的标签引用

 1 <!DOCTYPE html>
 2 <html>
 3 <body>
 4 
 5 <script>
 6 cars=["BMW","Volvo","Saab","Ford"];
 7 list:
 8 {
 9 document.write(cars[0] + "<br>"); 
10 document.write(cars[1] + "<br>"); 
11 document.write(cars[2] + "<br>"); 
12 break list;
13 document.write(cars[3] + "<br>"); 
14 document.write(cars[4] + "<br>"); 
15 document.write(cars[5] + "<br>"); 
16 }
17 </script>
18 
19 </body>
20 </html>

do-while

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4     <meta charset="UTF-8">
 5     <title>Document</title>
 6 </head>
 7 <body>
 8     <p>点击下面的按钮，主要i小于5就一直循环代码块</p>
 9     <button onclick="myFunction()">hit me</button>
10     <p id="demo"></p>
11     <script>
12     function myFunction()
13     {
14         var x="",i=0;
15         do{
16             x=x+"The number is " +i+ "<br />";
17             i++;
18         }        
19         while(i<5)
20          document.getElementById("demo").innerHTML=x;
21         
22     }
23     </script>
24     
25 </body>
26 </html>

结果：

while

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4     <meta charset="UTF-8">
 5     <title>Document</title>
 6 </head>
 7 <body>
 8     <p>点击下面的按钮，只要i小于5就一直循环代码块</p>
 9     <button onclick="myFunction()">hit me</button>
10     <p id="demo"></p>
11     <script>
12     function myFunction()
13     {
14         var x="",i=0;
15         while(i<5)
16         {
17             x=x+"The number is " +i+ "<br />";
18             i++;
19         }
20         document.getElementById("demo").innerHTML=x;
21     }
22     </script>
23 </body>
24 </html>

结果
：

for

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4     <meta charset="UTF-8">
 5     <title>Document</title>
 6 </head>
 7 <body>
 8     <script>
 9     cars=["BMW","Volvo","Saab","Ford"];
10     var i=0;
11     for(;cars[i];)
12     {
13         document.write(cars[i]+"<br />");
14         i++;
15     }
16     </script>
17 </body>
18 </html>

结果
：

break

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4     <meta charset="UTF-8">
 5     <title>Document</title>
 6 </head>
 7 <body>
 8     <p>点击按钮，测试带有break语句的循环</p>
 9     <button onclick="myFunction()">hit me</button>
10     <p id="demo"></p>
11     <script>
12     function myFunction()
13     {
14         var x="",i=0;
15         for(i=0;i<10;i++)
16         {
17             if(i==3)
18             {
19                 break;
20             }
21             x=x+"The numbe is "+i+ "<br />";
22 
23         }
24         document.getElementById("demo").innerHTML=x;
25     }
26     </script>
27 </body>
28 </html>

continue

 1 <!DOCTYPE html>
 2 <html lang="en">
 3 <head>
 4     <meta charset="UTF-8">
 5     <title>Document</title>
 6 </head>
 7 <body>
 8     <p>点击下面的按钮来执行循环，该循环会跳过i=3的步进</p>
 9     <button onclick="myFunction()">hit me</button>
10     <p id="demo"></p>
11     <script>
12     function myFunction()
13     {
14         var x="",i=0;
15         for(i=0;i<10;i++)
16         {
17             if(i==3)
18             {
19                 continue;
20             }
21             x=x+"the numeber is "+i +"<br />";
22             document.getElementById("demo").innerHTML=x;
23         }
24 
25     }
26     </script>
27 </body>
28 </html>

结果：

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