Aggregating keys with the same value in javascript objects

Question

I have an array of objects like so:

[{state: CA, value: 1}, {state: CA, value: 1}, {state: NY, value: 10}]

I would like the final result to be an object that contains the aggregates of the value for the same states:

{CA: 2, NY: 10}

I have this code below to do that:

let arr = [{state: 'CA', value: 1}, {state: 'CA', value: 1}, {state: 'NY', value: 10}];

let finalObj = {};

arr.map(function(item) {
  let state = item.state;
  if (!finalObj[state]) {
      finalObj[state] = item.value;
  }else {
      finalObj[state] += item.value;
  }
});

console.log(finalObj);

Is there a better way to do this rather than having those if and else checks inside of the map function?

Lux · Accepted Answer · 2018-09-16 23:14:54Z

First I recommend forEach instead of map if you want a simple loop. However in this case reduce is what you really want. I think this is the shortest way to do this I can think of:

const arr = [{state: 'CA', value: 1}, {state: 'CA', value: 1}, {state: 'NY', value: 10}];

const finalObj = arr.reduce((acc, item) => {
  acc[item.state] = (acc[item.state] || 0) + item.value;
  return acc;
}, {});

You can just replace the if/else with always using an assignment and just doing (acc[item.state] || 0) to always get a valid number.

Another option could be to only do the if part (which I think is a pretty clean solution):

const arr = [{state: 'CA', value: 1}, {state: 'CA', value: 1}, {state: 'NY', value: 10}];

const finalObj = arr.reduce((acc, item) => {
  if(!acc[item.state]) {
    acc[item.state] = 0;
  }
  acc[item.state] += item.value;
  return acc;
}, {});

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