if=============================

var score = prompt("请输入小明的期末成绩")
             score = parseInt(score)
             if (score==100) {
                alert('奖励一辆BMW');
             }
             else if ( score>=80 && score<=99) {
                alert('奖励一辆iPhone15s');
             }
             else if ( score>=60 && score<80) {
                alert('奖励一本参考书');
             }
             else{
                alert('继续努力');
             }

var height = prompt('请输入身高：')
             var money = prompt('请输入money：')
             var shuai = prompt('请输入shuai：')


             if(height>=180 &&  money>=100000000 && shuai>=500){
                alert('嫁给他');
             }
             else if(height>=180 ||  money>=100000000 || shuai>=500){
                alert('嫁吧，比上不足，比下有余');
             }
             else{
                alert('不嫁');
             }

var num1 = prompt()
            var num2 = prompt()
            var num3 = prompt()
            num1 = parseInt(num1)
            num2 = parseInt(num2)
            num3 = parseInt(num3)
            var temp;
            if (num1>num2) {
                temp =num1;
                num1 = num2;
                num2 = temp
            }
            if (num1>num3) {
                temp = num1;
                num1 = num3;
                num3 = temp
            }
            if (num2>num3) {
                temp = num2;
                num2 = num3;
                num3 = temp
            }
            alert('num1 num2 num3')

switch==============================

var score = prompt('请输入分数：');

            score = parseInt(score);

            switch(true){
                case (score>=60&&score<=100):
                    alert('合格');
                    break
                case(score>=0&&score<60):
                    alert('不合格');
                    break    
                default:
                    alert('请重新输入');
                    break
             }

var today = prompt('请输入参数：');
            today = parseInt(today);
            switch(today){
            case 1:
                alert('星期一');
                break;//结束整个switch语句
            case 2:
                alert('星期二');
                break;
            case 3:
                alert('星期三');
                break;
            case 4:
                alert('星期四');
                break;
            case 5:
                alert('星期五');
                break;
            case 6:
                alter('星期六');
                break;
            case 7:
                alert('星期日');
                break;
            default:
                alert('非法参数');
                break;//最后一个default可以不写break，但建议写上
        }

while=============================

var i = 1000;
        var num =0;
        while(i<=5000){
            i +=i * 0.05;
            num++;


        }
        alert(num);
    </script>

while(true){
            var score = prompt('请输入分数');
            if (score>=0 && score<=100) {
                alert('输入合法')
                break
            }else{
                alert('输入不合法')

            }

        }

for===============================

var sum = 0;
        for(var i = 1; i <= 100; i++){
            if(i % 2!= 0){
                sum += i;
            }   
        }
        document.write(sum+"<br>");

var sum = 0;
        var alist=[];
        for(var i = 1; i <= 100; i++){
            if(i % 7 == 0){
                alist.push(i);
                sum+=i;
            }
            
        }
        document.write(sum+"<br>");
        document.write(alist.length);

for(var i = 100; i < 1000; i++){
            if (Math.pow(parseInt(i/100),3)+Math.pow((parseInt(i/10)%10),3)+Math.pow((i%10),3) == i){
                
                    alert(i)

            }
        }
        

var num = prompt("请输入一个数");
        var count = 1; 
        for (; ;) { 
            if(num==""){ 
                alert("不能为空") 
                break 
        } 
        if(num/parseInt(num)!=1){ 
            alert("请不要输入小数") 
            break 
        } 
        if(num<0){ 
            alert("请输入正整数") 
            break 
         } 
        for (var i = 2; i <= parseInt(num / 2); i++) { 
            if (num % i == 0) { 
                count = 0; 
                break 
            } 
        } 
        if (num >= 2 && count == 1) { 
            alert(parseInt(num) + "是质数") 
        } else { 
            alert(num + "不是质数") 
        } 
        break
    }

step();
        function step(){
            for(i=1;i<=9;i++){
                for(j=1;j<=i;j++){
                document.write(i+"x"+j+"="+i*j +" ");
                }document.write("<br/>")
            }
        }

var flag=0;
        for(var j=1;j<101;j++){
            for(var i=2;i<j;i++){
                if(j%i==0){
                    flag=1;
                    break;
                }else{
                    flag=0;
                }
            }
            if(flag==0&&j!=1){
                console.log(j);
            }
        

计算器

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>计算器</title>
    <style type="text/css">
        .box{
            text-align: center;
        }
    </style>
    <script type="text/javascript">
                 window.onload = function () {
                    var value1 = document.getElementById("value1"); 
                    var value2 = document.getElementById("value2"); 
                    var value3 = document.getElementById("value3"); 
                    var btn = document.getElementById("btn"); 
                    var select = document.getElementById("select") 
                    btn.onclick = function () { 
                        var one, two, bignum; 
                        try { 
                            one = value1.value.split(".")[1].length 
                        } catch (e) { 
                            one = 1 
                        } 
                        try { 
                            two = value2.value.split(".")[1].length 
                        } catch (e) { 
                            two = 1 
                        } 
                        bignum = Math.pow(10, Math.max(one, two))
                        if (value1.value == "" || value2.value == "") { 
                            alert('值不能为空') 
                            value1.value = ""; 
                            value2.value = ""; 
                            value3.value = ""; 
                        } else if ((value1.value != parseFloat(value1.value)) || (value2.value != parseFloat(value2.value))) { 
                            alert("输入的值有误") 
                            value1.value = ""; 
                            value2.value = ""; 
                            value3.value = ""; 
                        } else { if (select.value == "+") { value = (parseFloat(value1.value) * bignum + parseFloat(value2.value) * bignum) / bignum value3.value = value 
                        } else if (select.value == "-") { value = (parseFloat(value1.value) * bignum - parseFloat(value2.value) * bignum) / bignum value3.value = value 
                        } else if (select.value == "*") { value = (parseFloat(value1.value) * bignum * parseFloat(value2.value) * bignum) / (bignum * bignum) value3.value = value 
                        } else { if (parseFloat(value2.value) == 0) { alert("除数不能为零") value1.value = ""; value2.value = ""; value3.value = ""; 
                        } else { 
                            value = ((parseFloat(value1.value) * bignum) / (parseFloat(value2.value) * bignum)) value3.value = value 
                        } 
                    } 
                } 
            }
        }


    </script>
</head>
<body>
    <div class="box">
    <h1>计算器</h1>
    <input type="text" id="value1">
    <select id="select">
    <option>+</option>
    <option>-</option>
    <option>*</option>
    <option>/</option>
    </select>
    <input type="text" id="value2">
    <input type="button" id="btn" value="=">
    <input type="text" id="value3">
    </div>

</body>
</html>