Group by count of objects within an array in Vanilla Javascript

Question

I have an array of objects:

[{person:101, year: 2012}, {person:102, year: 2012}, {person:103, year: 2013}]

And I want to be able to return an

Answer 1

As others have mentioned, an object would be a better fit for aggregating the data. You could use a normal loop, or reduce to do it:

var data = [{person:101, year: 2012}, {person:102, year: 2012}, {person:103,
year: 2013}];

var yearCounts = data.reduce(function (result, person) {
  var currentCount = result[person.year] || 0;
  result[person.year] = currentCount + 1;
  return result;
}, {});

console.log(yearCounts);

If you really need it as an array, you could the loop over the object and convert it to an array:

var data = [{person:101, year: 2012}, {person:102, year: 2012}, {person:103,
year: 2013}];

var yearCounts = data.reduce(function (result, person) {
  var currentCount = result[person.year] || 0;
  result[person.year] = currentCount + 1;
  return result;
}, {});

var year,
  yearCountArray = [];
for (year in yearCounts) {
  if (yearCounts.hasOwnProperty(year)) {
    yearCountArray.push({
      year: year,
      count: yearCounts[year]
    });
  }
}

console.log(yearCountArray);