javascript - Reduce Multiple Objects into One, Adding Values Together - Stack Overflow

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Let's say I have an array of objects:

[
    {
        category: 'Category A',
        mainUnits: 5,
        subUnits: 3
    },

    {
        category: 'Category A',
        mainUnits: 9,
        subUnits, 12
    },

    {
        category: 'Category B',
        mainUnits: 4,
        subUnits, 6
    },

    {
        category: 'Category B',
        mainUnits: 2,
        subUnits, 4
    }
]

However, I can't seem to get an array that looks like this:

[
    {
        category: 'Category A',
        mainUnits: 14,
        subUnits: 15
    },

    {
        category: 'Category B',
        mainUnits: 6,
        subUnits: 10
    }
]

I have been able to make some sort of headway, but can't quite get the exact result I need. Any guidance is greatly appreciated, thanks!

Let's say I have an array of objects:

[
    {
        category: 'Category A',
        mainUnits: 5,
        subUnits: 3
    },

    {
        category: 'Category A',
        mainUnits: 9,
        subUnits, 12
    },

    {
        category: 'Category B',
        mainUnits: 4,
        subUnits, 6
    },

    {
        category: 'Category B',
        mainUnits: 2,
        subUnits, 4
    }
]

However, I can't seem to get an array that looks like this:

[
    {
        category: 'Category A',
        mainUnits: 14,
        subUnits: 15
    },

    {
        category: 'Category B',
        mainUnits: 6,
        subUnits: 10
    }
]

I have been able to make some sort of headway, but can't quite get the exact result I need. Any guidance is greatly appreciated, thanks!

• javascript
• arrays
• object

Share Improve this question Follow edited Feb 28, 2018 at 0:29 anonymoose 1,24344 gold badges2525 silver badges6464 bronze badges asked Feb 28, 2018 at 0:10 LushmoneyLushmoney 4981212 silver badges2828 bronze badges

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3 Answers 3

Sorted by: Reset to default 6

The .reduce array method seems like a good fit here.

Use the array .find method to see if there is already an item with that category in the result. If so, add the fields, but if not, add the item to the result:

const input = [{
  category: 'Category A',
  mainUnits: 5,
  subUnits: 3
}, {
  category: 'Category A',
  mainUnits: 9,
  subUnits: 12,
}, {
  category: 'Category B',
  mainUnits: 4,
  subUnits: 6,
}, {
  category: 'Category B',
  mainUnits: 2,
  subUnits: 4,
}];

const result = input.reduce((result, item) => {
  const existing = result.find(x => x.category === item.category);
  if (existing) {
    existing.mainUnits += item.mainUnits;
    existing.subUnits += item.subUnits;
  } else {
    result.push(item);
  }

  return result;
}, []);

console.log(result);

@CRice's answer is great, but here's another approach if you'd like:

const input = [{
  category: 'Category A',
  mainUnits: 5,
  subUnits: 3
}, {
  category: 'Category A',
  mainUnits: 9,
  subUnits: 12
}, {
  category: 'Category B',
  mainUnits: 4,
  subUnits: 6
}, {
  category: 'Category B',
  mainUnits: 2,
  subUnits: 4
}];

let sums = {};
for (let entry of input) {
  sums[entry.category] = sums[entry.category] || {
    mainUnits: 0,
    subUnits: 0
  };
  sums[entry.category].mainUnits += entry.mainUnits;
  sums[entry.category].subUnits += entry.subUnits;
}

const result = Object.keys(sums).map(category => Object.assign({
  category
}, sums[category]));

console.log(result);

You need a function like this:

function catTotals(catsArray){
  var cats = [], totals = [];
  for(var i=0,ca,c,o,x,l=catsArray.length; i<l; i++){
    ca = catsArray[i]; c = ca.category; x = cats.indexOf(c);
    if(x === -1){
      cats.push(c); totals.push({category:c, mainUnits:ca.mainUnits, subUnits:ca.subUnits});
    }
    else{
      o = totals[x]; o.mainUnits += ca.mainUnits; o.subUnits += ca.subUnits;
    }
  }
  return totals;
}
var cats = [
    {
        category: 'Category A',
        mainUnits: 5,
        subUnits: 3
    },

    {
        category: 'Category A',
        mainUnits: 9,
        subUnits: 12
    },

    {
        category: 'Category B',
        mainUnits: 4,
        subUnits: 6
    },

    {
        category: 'Category B',
        mainUnits: 2,
        subUnits: 4
    }
]
console.log(catTotals(cats));

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