1、Math

• Math.ceil() —>上取整
• Math.floor() —下取整
• Math.round() —四舍五入取整
• Math.max() —取最大值
• Math.min() —取最小值

• Math.random —取0<=num<1的随机数

  <script>
        var num = 13.45;
        /* Math.ceil()  -->上取整 */ 
        console.log(Math.ceil(num));
        /* Math.floor() --下取整 */ 
        console.log(Math.floor(num));
        /* Math.round() --四舍五入取整 */ 
        console.log(Math.round(13.56));
        /* Math.max()   --取最大值*/
        console.log(Math.max(1,3,5,6));
        /* Math.min()   --取最小值 */ 
        console.log(Math.min(1,3,5,6));
        /* Math.random  --取0<=num<1的随机数 */ 
        console.log(Math.random());
    </script>

  

  2、Date

  / 年月日 /
  var date = new Date()
  / date.getFullYear() —年 /
  var year = date.getFullYear();
  / date.getMonth() —月份从0开始 /
  var month = date.getMonth()+1;
  / date.getDate() —日 /
  var date = date.getDate();

  <script>
        /* 年月日 */ 
        var date = new Date()
        console.log(date);
        /* date.getFullYear()   --年 */ 
        var year = date.getFullYear();
        console.log(year);
        /* date.getMonth()  --月份从0开始 */ 
        var month = date.getMonth()+1;
        console.log(month);
        /* date.getDate()   --日 */  
        var date = date.getDate();
        console.log(date);
    </script>

  

  例子

  1.根据输入的值显示对应格式的数字

  <script>
        function handleNum(value){
            if(value>=100000000){
                    value = Math.ceil(value/100000000)+"亿"
            }else if(value>10000){
                    value = Math.ceil(value/10000)+"万"
            }
            return value
        }
        var res = handleNum(2154578);
        console.log(res);
    </script>

  

  2.改变数组对象中的数字格式

  <script>
        function handleNum(value){
            if(value>=100000000){
                    value = Math.ceil(value/100000000)+"亿"
            }else if(value>10000){
                    value = Math.floor(value/10000)+"万"
            }
            return value+""
        }
        var arr = [
            /* 4000 */ 
            {name:"你好",playCount:4000},
            /* 40万 */ 
            {name:"它",playCount:400040},
            /* 2亿 */ 
            {name:"谁",playCount:200000000}
        ]
        var res = [];
        arr.forEach(item=>{
            item.playCount = handleNum(item.playCount);
        })
        console.log(arr);
    </script>

  

  3.实时显示时间

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

  

  4.计算当前时间与指定时间的时间差

  <script>
        var time = "2021-10-13 21:30:00"
        /* 1、获取当前时间的毫秒数 */ 
        var d = new Date();
        console.log(d.getTime());
        var startTime = d.getTime();
        /* 2、获取活动开始时间的毫秒 */
        var endTime = new Date(time).getTime();
        console.log(endTime);
        var gapTime = endTime - startTime;
        console.log(gapTime);
        var seconds = Math.floor((gapTime/1000)%60);
        var minutes = Math.floor((gapTime/60000)%60);
        var hours = Math.floor(gapTime/1000/3600);
        console.log(hours+":"+minutes+":"+seconds)
    </script>

  

  <script>
        function show(time){
            var d = new Date();
            // console.log(d.getTime());
            var startTime = d.getTime();
            /* 2、获取活动开始时间的毫秒 */
            var endTime = new Date(time).getTime();
            // console.log(endTime);
            var gapTime = endTime - startTime;
            // console.log(gapTime);
            var seconds = Math.floor((gapTime/1000)%60);
            var minutes = Math.floor((gapTime/60000)%60);
            var hours = Math.floor(gapTime/1000/3600)%24;
            var days = Math.floor(gapTime/3600000/24);
            console.log(days+"天"+hours+":"+minutes+":"+seconds)
        }
        show("2021-10-13 21:30:00");
        show("2021-10-19 08:30:00")
    </script>

  

  5.计算字符串记录的时间之差

  <script>
        function show(){
            var gapTime = endTime - startTime;
            // console.log(gapTime);
            var seconds = Math.floor((gapTime/1000)%60);
            var minutes = Math.floor((gapTime/60000)%60);
            var hours = Math.floor(gapTime/1000/3600)%24;
            var days = Math.floor(gapTime/3600000/24);
            console.log(days+"天"+hours+":"+minutes+":"+seconds)
        }
        var s = "2021:10:13,18:00:00"
        var startTime = s.split(",")[0]
        var endTime = 2021+":"+s.split(",")[1]
        console.log(startTime);
        console.log(endTime);
        startTime = new Date(startTime);
        endTime = new Date(endTime);
        console.log(startTime);
        console.log(endTime);
        show();
    </script>