JSFiddle - React, Tailwind, and code Playground

JavaScript

var obj = {'series1':[{'date1':4},{'date2':2},{'date3':6}],
'series2':[{'date1':5},{'date2':3},{'date3':4},{'date4':6}],
'series3':[{'date2':1},{'date4':9}]};
console.log(obj);
var output = {};
for(var series in obj ){
    var dateObjects = obj[series];
    for( var dateObj in dateObjects ){
        var dates = dateObjects[dateObj];
        for( var date in dates ){
         if( typeof output[date] == "undefined" ){
          output[date] = [];
         }
         output[date].push(dates[date]);
        }
    }
}
console.log(output);