JS - select structure

1. If structure

grammar:

 if (condition) {                                  JavaScript statement

                        }

                        This statement requires that the condition must be true before the JavaScript statement inside {} will be executed.

The condition here will only return boolean type is either true or false   

   <script>
        var result1 = 61;
        var result2 = 55;
        if (result1>60){
            alert('恭喜你，过关啦~') //页面弹出语句
        }
        if(result2>60){
            alert('恭喜你，过关啦~') //页面没有显示
        }
    </script>

---

2. Multiple if structures

grammar:

 if (condition 1) {                       JavaScript statement 1

} else if (condition 2) {                       JavaScript statement 2

}else{                      JavaScript statement 3

}

Here it is executed from top to bottom, if condition 1 is satisfied, execute JavaScript statement 1, and then the program stops; if condition 1 is not satisfied, then continue to compare condition 2, if it is satisfied, execute JavaScript statement 2, if not, continue to search; if the previous If none of the conditions are met, the JavaScript statement in the last else is output. 

 //示例1：
<script>
        // 多重if结构嵌套，按照顺序来判断，只要有一个为true，其他都不执行
        var result = prompt('请输入你的考试成绩')
        if (result>90){
            alert('优秀！')
        }else if(result>=70){
            alert('良好！')
        }
        else if(result>=60){
            alert('及格！')
        }else{
            alert('希望你下次再努力！')
        }
 </script>

//示例2：
  <script>
        var year = prompt('请输入年份' )
        if (year % 4==0 && year % 100 !=0 || year % 400==0){
            alert(year + '年是闰年')
        }
        else {
            alert(year + '年是平年')
        }
    </script>

---

Two, switch statement

The value in the switch (expression) must be consistent with the value and type of the case to compare

grammar:

  switch(expression) { 

                                case n: 

                                        code block  

                                        break;

                                case n:

                                        code block

                                        break;

                                default:

                                        default code block

                        }

 Case and default must be followed by a break to terminate the subsequent comparison

  <script>
        // switch(表达式)里面的值，要和 case 的值和类型保持一致才能比较
        switch(8){
            case 1 :
                document.write('结果为1');
                break;
            case 2 :
                document.write('结果为9');
                break;
            default:
                document.write('都不对!');
                break
        }
    </script>