JavaScript中思考do…while和while语句的区别

javascript文章/教程1年前 (2023)发布勤奋小助

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Do…while和while…do的区别在于对于临界值的测试上。当在执行的时候只选择临界值来测试时，你会发现do…while至少要执行一次，而while…do则一次都不会执行。但是，当不去考虑临界值的时候，do…while和while…do的执行结果是一样的。让我们用事实来说话吧：

只测试临界值：

 1 <!DOCTYPE html>
 2 <html>
 3 <body>
 4 <p>Try to practice while statement!</p>
 5 <p id = "demo"></p>
 6 <script>
 7     var x = "";
 8     var i = 0;
 9     var x = "";
10     var i = 0;
11     while(i < 0)
12     {
13         x += "The number is " + i + "." + "<br>";
14         i ++;
15         document.getElementById("demo").innerHTML = x;
16     }
17     
18     do
19     {
20         x +="The number is " + i + "." + "<br>";
21         i++;
22         document.getElementById("demo").innerHTML = x;
23     }
24     while(i < 0); 
25 </script>
26 </body>
27 </html>

View Code

尝试着只去测试临界值（i=0）时，你会发现只有do…while会输出，而while…do会在判断时直接不符合条件，这似乎很简单的逻辑，但有时候，也会让死钻牛角尖的我陷入混乱的状态。

不考虑临界值时只执行while语句：

 1 <!DOCTYPE html>
 2 <html>
 3 <body>
 4 <p>Try to practice while statement!</p>
 5 <p id = "demo"></p>
 6 <script>
 7     var x = "";
 8     var i = 0;
 9     while(i < 5)
10     {
11         x += "The number is " + i + "." + "<br>";
12         i ++;
13         document.getElementById("demo").innerHTML = x;
14     }
15 </script>
16 </body>
17 </html>

View Code

不考虑临界值时只执行do…while语句：

 1 <!DOCTYPE html>
 2 <html>
 3 <body>
 4 <p>Try to practice do while statement!</p>
 5 <p id = "demo"></p>
 6 <script>
 7     var x = "";
 8     var i = 0;
 9     do
10     {
11         x +="The number is " + i + "." + "<br>";
12         i++;
13         document.getElementById("demo").innerHTML = x;
14     }
15     while(i < 5);
16 </script>
17 </body>
18 </html>