示例：

1-1. 亿/万单位

<script>
        function handleNum(num){
            if(num>=100000000){
            num= num/100000000 +"亿"
        }
        else if (num>=10000){
            num =num/10000 +"万"
        } 
        return num+"";
    }
        var arr =[
            {name:"你好",playCount:4000},
            {name:"它",playCount:"400040"},
            {name:"谁",playCount:200000000}
        ]
        arr.forEach(item=>{
            item.playCount =handleNum(item.playCount);
        })
        console.log(arr)
    </script>

1-2. 时钟

    <div id="app"></div>
    <script>
        setInterval(show, 1000)
        function show() {
            var app = document.getElementById("app");
            var date = new Date();
            var hours = date.getHours();
            var minutes = date.getMinutes();
            var seconds = date.getSeconds();
            var sum = handleNum(hours) + ":" + handleNum(minutes) + ":" + handleNum(seconds);
            console.log(sum);
            app.innerHTML = sum;
        }
        function handleNum(num) {
            if (num < 10) {
                num = "0" + num;
            }
            return num;
        }
    </script>

1-3. 到达某时间点的倒计时

    <div id="app"></div>
    <script>
            setInterval(show,1000);
            function show(){
            var app = document.getElementById("app")
            var test = "2021:10:19,18:00:00";
            /* 转换为"2021-10-19 08:30:00" */
            var date =test.split(",")[0].replace(/\:/g,"-");
            // console.log(date)
            var t=[date,test.split(",")[1]];
            var time =t.join(" ");
            // console.log(time)
                /* 倒计时 */
            var d = new Date;
            /* 1.获取当前时间的毫秒数 */
            // console.log(d.getTime());
            var startTime = d.getTime();
            /* 2.获取活动时间的毫秒数 */
            var endTime = new Date(time).getTime();
            // console.log(endTime.getTime())
            /* 3.得到中间间隔的毫秒数 */
            var gapTime = endTime - startTime;
            // console.log(gapTime);
            var seconds = Math.floor(gapTime / 1000);
            // console.log(seconds);
            var day = Math.floor(seconds / (3600 * 24));
            // console.log(day);
            var hours = Math.floor(seconds / 3600%24);
            // console.log(hours)
            var minutes = Math.floor(seconds / 60%60);
            // console.log(minutes);
           var s =Math.floor(seconds%60)
            // console.log(seconds)
            var times = handleNum(day)+ ":" + handleNum(hours) + ":" + handleNum(minutes) + ":" + handleNum(s);
            console.log(times)
            app.innerHTML=times
        }
        function handleNum(num) {
            if (num < 10) {
                num = "0" + num;
            }
            return num;
        }    
    </script>

1-4. 进制转换

1. 十进制转二进制

   <script>
        var num = 10;
        var arr =[];
        while(num>0){
            arr.unshift(num%2);
            num = Math.floor(num/2);
        }
        console.log(Number(arr.join("")))
    </script>

2. 二进制转十进制

   <script>
        var num = 1010;
        /* 
        0*2^0+1*2^1+0*2^2+1*2^3
        0+2+0+8
         */
        /* [0,1,0,1] */
        var str = (num+"").split("").reverse();
        console.log(str)
        /* Math.pow(2,2) */
        // console.log(Math.pow(2,3))
        var sum = 0;
        str.forEach((item,index)=>{
            sum+= item*Math.pow(2,index);
        })
        console.log(sum)
    </script>