Aggregate records in JavaScript

Question

Given an array of objects representing a series of transactions i.e.

[{ person: 'a', sweets: 5 }, { person: 'b', sweets: 8 }, { person: 'a', sweets: 6 }]

How can I efficiently consolidate these records, to a total tally for each person i.e. :

[{ person: 'a', sweets: '11'}, { person: 'b', sweets: 8 }]

Is there some inbuilt function I donlt know about, or would I have to prototype one?

no there is no inbuilt function unless you are using a library like lodash or underscore. Not hard to do this yourself using a temporary object with person as keys — charlietfl
– charlietfl, Commented Oct 30, 2016 at 15:01
Have you tried anything so far? If so can you share your code? — Aurelio
– Aurelio, Commented Oct 30, 2016 at 15:22

Nina Scholz · Accepted Answer · 2016-10-30 15:17:21Z

2

You could use a hash table for grouping and summing.

var data = [{ person: 'a', sweets: 5 }, { person: 'b', sweets: 8 }, { person: 'a', sweets: 6 }],
    grouped = [];

data.forEach(function (a) {
    if (!this[a.person]) {
        this[a.person] = { person: a.person, sweets: 0 };
        grouped.push(this[a.person]);
    }
    this[a.person].sweets += a.sweets;
}, Object.create(null));

console.log(grouped);

answered Oct 30, 2016 at 15:17

Nina Scholz

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Comments

Daniel Oliveira · Accepted Answer · 2016-10-30 15:29:10Z

There isn´t any built in function. You have to use two loops. Firstly you need to loop through the original array and for each iteration of that loop you need to loop through the final array to find the correct position to update. I would do something like this (I did not test this solution):

var arrayStart = [{ person: 'a', sweets: 5 }, { person: 'b', sweets: 8 }, { person: 'a', sweets: 6 }]
var arraySum = [];

arrayStart.forEach(function(valueStart, indexStart){
  var found = false;
  for(var i = 0; i < arraySum.length; i++){
    if(arraySum[i].person == valueStart.person){
      found = true;
      arraySum[i].sweets += valueStart.sweets;
      break;
    }
  } 
  if(!found){
    arraySum.push(valueStart);
  }
});

console.log(arraySum);

You can also use @NinaScholz solution, using an Hash-table prevents the second loop, basically it is more efficient.

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