JS获取昨天/今天/今年第一天的方法

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//获取昨天

            var day1 = new Date();

            day1.setTime(day1.getTime()-24*60*60*1000);

            var yesterDate = day1.getFullYear()+"/" + (day1.getMonth()+1) + "/" + day1.getDate();

 //获取当前日期

            var date = new Date();

            var year = date.getFullYear();

            var month = date.getMonth() + 1;

            var day = date.getDate();

            if (month < 10) {

                month = "0" + month;

            }

            if (day < 10) {

                day = "0" + day;

            };

            var currentdate = year + '-' + month + '-' + day;

//获取今年第一天

            var date1 = new Date();

            var year1 = date.getFullYear();

            var firstMonth = year1 + '-' + '01' + '-' + '01';