stocksReturnsByTenYearWindow.js

var fs = require("fs");

var gatherStockData = function(stockData){
 parsedData = JSON.parse(stockData)
 return parsedData
}

var sortByDecade = function(fileData){
    // expected input is filedata = [{"Year": 1928,"S&P 500": "43.81%"}]

  var tenPercentBreakdowns = new Map();

  fileData.forEach(function(v){
		// var roundedValue = Math.round(parseFloat(v["S&P 500"]) / 10) * 10; // rounds to tens place
    var roundedValue = Math.round(parseFloat(v["S&P 500"])); // rounds to single digit
    var obj =  tenPercentBreakdowns.get(roundedValue);
    if(obj === undefined)
    {
      obj = { 'percentWindow': roundedValue, 'count': 0 };
      tenPercentBreakdowns.set(roundedValue, obj);
    }

    obj.count++;
  });

  return Array.from(tenPercentBreakdowns.values()).sort(function(a,b){
		return a.percentWindow - b.percentWindow;
	});
}

var fileData = gatherStockData(fs.readFileSync("stockdata.json"));

output = sortByDecade(fileData)
console.log(output)