需求来源：后端返回所有时间数组和数值数组，前端按天、周、月进行统计用于图表的展示。

实现效果如下：原始数据是两个数组，date数组存放日期，value数组存放日期对应的数值，经过按周分组和按月分组输出week和month（2020-02-27是周四，2020-03-02是周一，2020-03-09也是周一，符合要求）

封装了两个方法，代码如下：groupByWeek和groupByMonth，输入值均为date-日期数组和value-值数组

const date = ["2020-02-27","2020-02-28","2020-02-29","2020-03-01", "2020-03-02", "2020-03-03", "2020-03-04", "2020-03-05", "2020-03-06", "2020-03-07", "2020-03-08", "2020-03-09", "2020-03-10", "2020-03-11", "2020-03-12", "2020-03-13", "2020-03-14", "2020-03-15", "2020-03-16", "2020-03-17", "2020-03-18", "2020-03-19", "2020-03-20", "2020-03-21"];
    const value = [0,0,0,3, 2, 2, 6, 5, 3, 5, 4, 8, 5, 8, 9, 9, 0,4,6,7,8,1,2,3];
    console.log({date});
    console.log({value});
    const week = groupByWeek(date, value);
    const month = groupByMonth(date, value);
    console.log({week});
    console.log({month});

    // 按周分组
    function groupByWeek(date, value) {
        const weekDay = ["星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六"];
        const weekDate = [];
        let newDate = [];
        let newValue = [];
        /*遍历，把所有日期转为星期X*/
        date.forEach((item) => {
            const myDate = new Date(Date.parse(item));
            weekDate.push(weekDay[myDate.getDay()]);
        });
        /*从前往后找第一个“星期日”，返回下标*/
        const index = weekDate.findIndex((value)=>{
            return value=="星期日";
        });
        if(index >= 0){ //日期数组内可以找到"星期日"
            /*以周为单位开始分组*/
            // 第一个周为一个数组，其他的按7天开始分组
            const date1 = date.slice(0,index+1);
            const date2 = date.slice(index+1,date.length);
            const value1 = value.slice(0,index+1);
            const value2 = value.slice(index+1,value.length);
            newDate.push(date1[0]);
            newValue.push(eval(value1.join("+")));
            const result = dataGroupFunc(date2, value2, 7);
            newDate = newDate.concat(result.newArr1);
            newValue = newValue.concat(result.newArr2);
        } else { //日期数组内找不到"星期日"
            newDate = date;
            newValue = [eval(value.join("+"))];
        }
        return {
            newDate: newDate,
            newValue:newValue
        };
        // 图表数据分组方法:arr1-图表横坐标，arr2-图表纵坐标数据，group-以几条数据为一组
        function dataGroupFunc(arr1, arr2, group) {
            const newArr1 = [];
            const newArr2 = [];
            for(let i = 0; i < arr1.length;) {
                newArr1.push(arr1[i]);
                let count = 0;
                for (let j = i; j < i+group; j++) {
                    if (arr2[j]){
                        count += arr2[j];
                    }
                }
                newArr2.push(count);
                i+=group;
            }
            return {
                newArr1: newArr1,
                newArr2: newArr2
            }
        }
    }

    // 按月分组
    function groupByMonth(date, value) {
        const newDate = [];
        const newValue = [];
        const newDate1 = [];
        const newValue1 = [];
        const monthArr = [];
        const indexArr = [];
        /*遍历，把所有日期的月份取出*/
        date.forEach((item) => {
            item.split('-');
            monthArr.push(item.split('-')[1]);
        });
        group(monthArr, 0 , 0);
        /*根据分好组的下标信息开始截取原始数组*/
        indexArr.forEach((item) => {
            newDate1.push(date.slice(item[0],item[item.length-1]+1));
            newValue1.push(value.slice(item[0],item[item.length-1]+1));
        });
        /*把每个分组的第一项拿出用于图表横坐标的值*/
        newDate1.forEach((item) => {
            newDate.push(item[0]);
        });
        /*计算纵坐标的和值*/
        newValue1.forEach((item) => {
            newValue.push(eval(item.join("+")));
        });
        return {
            newDate: newDate,
            newValue: newValue
        };
        //数组相同项合一组记录下标
        function group(arr, index, index1) {
            if (index < arr.length) {
                indexArr[index1] = [index];
                for(let i=index+1; i<arr.length; i++){
                    if (arr[i] == arr[index]){
                        indexArr[index1].push(i);
                    } else {
                        group(arr, i, index1+1);
                        break;
                    }

                }

            }
        }
    }