按香草Javascript中数组中对象的数量进行分组

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 aggregate count for each year (much like a group by in SQL would achieve) in the format of:

[{year: 2012, count:2}, {year: 2013, count:1}]

What is the best way to achieve this in Vanilla Javascript?

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);

Answer 2

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); 

Answer 3

Use a generic group by key reducer that will count the number of items in each group. I will take inspiration from a previous answer of mine . This function will return another function that act as a reducer, but you can always give it the key that you want as a parameter.

 const groupByCounter = key => (result,current) => { const item = Object.assign({},current); if (typeof result[current[key]] == 'undefined'){ result[current[key]] = 1; }else{ result[current[key]]++; } return result; }; const data = [{person:101, year: 2012}, {person:102, year: 2012}, {person:103, year: 2013}]; const group = data.reduce(groupByCounter('year'),{}); console.log(group);