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

You need to use simple for like below :

var data = [{person:101, year: 2011}, 
            {person:102, year: 2012}, 
            {person:103, year: 2011}];


var result = {},
  i;

for(i=0; i < data.length; i++){
  let year = data[i].year;
  if(typeof result[year] === 'undefined'){
    result[year] = 1;
  }
  else{
    result[year] = result[year] + 1;
  }

}

console.log(result);