How to sum values in array by specific object key?

Question

I have an array of js objects which look like this:

var objArr = [
     {"Country": "US", "City": "MyCity1", "2017-07-12": "1", "2017-07-13": "2"},
     {"Country": "US", "City": "MyCity2", "2017-07-12": "2", "2017-07-13": "2"},
     {"Country": "CN", "City": "MyCity1", "2017-07-12": "5", "2017-07-13": "7"},
     {"Country": "CN", "City": "MyCity2", "2017-07-12": "5", "2017-07-13": "7"}
   ]

I wanna create a new array where the country is unique, but also the dates should be summed.

{"Country": "US", "2017-07-12":  "3", "2017-07-13": "4"},
{"Country": "CN", "2017-07-12": "10", "2017-07-13": "14"}
            ^^^^                ^^^^                ^^^^   

I have problems getting my head around it. Do I have to filter first, reduce it somehow, remap it,... I've no idea to start? Any help would be appreciated, thank you!

Answer 1

Use Array.reduce, check if an object with the current country exists, if it does, loop through the Object.keys to update the values, if not, push a new one :

EDIT : If you don't want City you can destructure it {City, ...curr}

var objArr = [
  { Country: "US", City: "MyCity1", "2017-07-12": "1", "2017-07-13": "2" },
  { Country: "US", City: "MyCity2", "2017-07-12": "2", "2017-07-13": "2" },
  { Country: "CN", City: "MyCity1", "2017-07-12": "5", "2017-07-13": "7" },
  { Country: "CN", City: "MyCity2", "2017-07-12": "5", "2017-07-13": "7" }
];

var result = objArr.reduce((acc, {City, ...curr}) => {
  const ndx = acc.findIndex(e => e.Country === curr.Country);
  if (ndx > -1) {
    const [country, ...keys] = Object.keys(curr);
    keys.forEach(k => {
      acc[ndx][k] = +acc[ndx][k] + +curr[k];
    });
  } else {
    acc.push(curr);
  }
  return acc;
}, []);

console.log(result);