一、JavaScript流程控制语句
1.if... else语句
if (条件表达式) {语句;} else if (条件表达式) {语句;} ... else {语句;}
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - if语句</title> <script type="text/javascript"> var a = 90; if(a>80){ alert("很好"); }else if(a>60){ alert("及格"); }else{ alert("很差") } </script> </head> <body> </body> </html>
2.switch语句
多重分支语句:
switch () { case n : ...};
switch语句是多重条件判断,用于多个值相等的比较。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - switch语句</title> <script type="text/javascript"> var a = 80; switch (a){ case 80: alert("很好"); //判断条件两个值是否相等,相等则执行 break; case 60: alert("及格"); break; default: //在匹配的时候,条件都不满足执行 alert("不好"); } </script> </head> <body> </body> </html>
3.do...while语句
do...while语句是一种先运行,后判断的循环语句。也就是说,不管条件是否满足,至少先运行一次循环体。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - do...while语句</title> <script type="text/javascript"> //先运行,后判断 var i = 1; do{ alert(i); i++; }while(i<=5); </script> </head> <body> </body> </html>
4.while语句
while语句是一种先判断,后运行的循环语句。也就是说,必须满足条件了之后,方可运行循环体。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - while语句</title> <script type="text/javascript"> //先判断,后运行 var i = 1; while (i<=5) { alert(i); i++; } </script> </head> <body> </body> </html>
5.for语句
for语句也是一种先判断,后运行的循环语句。但它具有在执行循环之前初始变量和定义循环后要执行代码的能力。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - for语句</title> <script type="text/javascript"> //先判断,后运行 for(var i=1;i<=5;i++){ alert(i) } </script> </head> <body> </body> </html>
(6)for...in语句
for...in语句是一种精准的迭代语句,可以用来枚举对象的属性。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - for...in语句</title> <script type="text/javascript"> //弹框弹出所有的key值 var box={ "name":"suner", //键值对,左边是属性名,右边是值 "addr": "wuxi", "age":20 }; for(var p in box){ //列举出对象的所有的属性 alert(p); } </script> </head> <body> </body> </html>
break和continue语句用于在循环中精确地控制代码的执行。其中,break语句会立即退出循环,强制继续执行循环体后面的语句。而continue语句退出当前循环,继续后面的循环。
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - for...break语句</title> <script type="text/javascript"> for (var box = 1; box <= 10; box++) { if (box == 5) break; //如果box是5,就退出循环 document.write(box); document.write('<br />'); } </script> </head> <body> </body> </html>
<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>js - for...break语句</title> <script type="text/javascript"> for (var box = 1; box <= 10; box++) { if (box == 5) continue; //如果box是5,就退出当前循环,继续下次循环 document.write(box); document.write('<br />'); } </script> </head> <body> </body> </html>