Aggregate and average values inside an object Javascript

Question

I have an object in the form of:

{
"Dates": ["January", "January", "March", "March", "March", "November", "November"],
"Values": [45.6, 0.5, 59.3, 46.56, 2.21, 48.59, 5.5]
}

What I need is to aggregate the values in "Values" (average) by month

The object I am seeking:

{
"Dates":["January", "March", "November"],
"Values":[23.05, 36.02, 27]
}

I know there have been several posts about it here. But I am not sure how to apply it to an object with arrays. Would appreciate very much any ideas. Thanks

Loop through the two arrays and turn it into something like `{"January": [45.6, 0.5], "March": [59.3, 46.55, 2.21], ...}`. Then it will be easy to calculate the averages of each. — Barmar, Dec 09 '22 at 16:57

score 1 · Answer 1 · answered Dec 09 '22 at 17:05

Something like this should work.

// First reduce the data to an object with the dates as keys
const grouped = data.Dates.reduce((acc, date, i) => {
  if (acc[date]) {
    acc[date].push(data.Values[i])
  } else {
    acc[date] = [data.Values[i]]
  }
  return acc
}, {});

// Then reduce the object to the desired format
const result = Object.keys(grouped).reduce((acc, date) => {
  acc.Dates.push(date)
  acc.Values.push(grouped[date].reduce((a, b) => a + b) / grouped[date].length)
  return acc
}, {Dates: [], Values: []});

score 0 · Answer 2 · answered Dec 09 '22 at 17:10

You could try something like this.

const obj = {
  "Dates": ["January", "January", "March", "March", "March", "November", "November"],
  "Values": [45.6, 0.5, 59.3, 46.56, 2.21, 48.59, 5.5]
}

// new Set() will remove all duplicated items
const months = [...new Set(obj.Dates)];

const result = {
  "Dates": months,
  "Values": months.map(month => {
    // find values for given month
    const values = obj.Values.filter((_, i) => obj.Dates[i] === month);
    
    // sum the values and divide by the amount
    return values.reduce((a, b) => a + b) / values.length;
  })
}

console.log(result);

Bdobry, huge thanks, also to @chribjel for both of your prompt answers. Both solutions give the intended result . — Cam, Dec 09 '22 at 17:25

score 0 · Answer 3 · answered Dec 09 '22 at 17:54

You could split the tasks into smaller parts and use functions for

grouping,
getting an average of an array
getting the complete result by taking an object of groups.

const
    getAverage = array => array.reduce((a, b) => a + b) / array.length,
    groupBy = (keys, values) => {
        const groups = {};

        for (let i = 0; i < keys.length; i++)
            (groups[keys[i]] ??= []).push(values[i]);

        return groups;
    },
    getAverages = groups => Object.entries(groups).reduce((r, [month, values]) => {
        r.Dates.push(month);
        r.Values.push(getAverage(values));
        return r;
    }, { Dates: [], Values: [] }),
    data = { Dates: ["January", "January", "March", "March", "March", "November", "November"], Values: [45.6, 0.5, 59.3, 46.56, 2.21, 48.59, 5.5] },
    result = getAverages(groupBy(data.Dates, data.Values));

console.log(result);

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Aggregate and average values inside an object Javascript

3 Answers3