JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/dayjs/1.10.7/dayjs.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/dayjs/1.10.7/plugin/isoWeek.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/dayjs/1.10.7/plugin/customParseFormat.min.js"></script>

JavaScript

dayjs.extend(window.dayjs_plugin_isoWeek);
dayjs.extend(window.dayjs_plugin_customParseFormat);

let data =[
    {"Day": "12-06-2021", "ROAS": 17.3, "ConversionRate": 0.1472, "VisitRate": 0.0488},
    {"Day": "12-07-2021", "ROAS": 14.79, "ConversionRate": 0.1251, "VisitRate": 0.0555},
    {"Day": "12-08-2021", "ROAS": 17.12, "ConversionRate": 0.1358, "VisitRate": 0.052},
    {"Day": "12-09-2021", "ROAS": 14.95, "ConversionRate": 0.1391, "VisitRate": 0.0537},
    {"Day": "12-10-2021", "ROAS": 19.85, "ConversionRate": 0.1394, "VisitRate": 0.0853},
    {"Day": "12-11-2021", "ROAS": 25.03, "ConversionRate": 0.1565, "VisitRate": 0.1033},
    {"Day": "12-12-2021", "ROAS": 24.58, "ConversionRate": 0.1528, "VisitRate": 0.1172},
    {"Day": "12-13-2021", "ROAS": 14.2, "ConversionRate": 0.1517, "VisitRate": 0.0481},
    {"Day": "12-14-2021", "ROAS": 14.32, "ConversionRate": 0.1564, "VisitRate": 0.0488},
    {"Day": "12-15-2021", "ROAS": 12.54, "ConversionRate": 0.1436, "VisitRate": 0.0473},
    {"Day": "12-16-2021", "ROAS": 14.47, "ConversionRate": 0.1574, "VisitRate": 0.0442},
    {"Day": "12-17-2021", "ROAS": 18.2, "ConversionRate": 0.1283, "VisitRate": 0.0729},
    {"Day": "12-18-2021", "ROAS": 72.93, "ConversionRate": 0.1366, "VisitRate": 0.2267},
    {"Day": "12-19-2021", "ROAS": 72.29, "ConversionRate": 0.1638, "VisitRate": 0.1715}
];

let average = {};

let currentWeek = 0
let i = 0;

data.forEach(day => {
	let week = dayjs(day.Day, "MM-DD-YYYY").isoWeek();
  let averageWeek = average[[week]];
  if (averageWeek) {
    average = {...average, [week]: {"ROAS": day.ROAS + averageWeek.ROAS, "ConversionRate": day.ConversionRate + averageWeek.ConversionRate, "VisitRate": day.VisitRate + averageWeek.VisitRate, count: averageWeek.count + 1}};
  }
  else {
  	average = {...average, [week]: {"ROAS": day.ROAS, "ConversionRate": day.ConversionRate, "VisitRate": day.VisitRate, count: 1}};
  }
  
});

let newData = [];
Object.entries(average).forEach(
    ([key, value]) => {
   ...