Question 270325 in Javascript Question posted in
A very good W3school tutorial can be found here.

Total JavaScript Array by one Property

I’ve looked at lots of examples but cannot get this to work. At best, I end up with a version of the existing array with properties replicated that should be lost in the summary.

I would like to output a new array of totals for each location and not include any details for specific periods, the period property etc.
I think my issue is with Object.assign({}, o); where I am creating a full copy, but I can’t see how to just create an object with just the location, to which the totals can be added.

This is a simplified example and in the original I am summing multiple columns and dropping multiple columns.

let helper = {};
let locationPeriodCounts = [
  {"period": "2023-10-21", "location": "228", "countIn": 6},
  {"period": "2023-10-22", "location": "228", "countIn": 8},
  {"period": "2023-10-23", "location": "228", "countIn": 3},
  {"period": "2023-10-24", "location": "228", "countIn": 1},
  {"period": "2023-10-21", "location": "229", "countIn": 5},
  {"period": "2023-10-22", "location": "229", "countIn": 18},
  {"period": "2023-10-23", "location": "229", "countIn": 8},
  {"period": "2023-10-24", "location": "230", "countIn": 3},
  {"period": "2023-10-25", "location": "230", "countIn": 4}
];

let locationCounts = locationPeriodCounts.reduce(function(r, o) {
  let key = o.location;
  if (!helper[key]) {
    helper[key] = Object.assign({}, o); // create a copy of o
    helper[key].totalCount = 0;
    r.push(helper[key]);
  } else {
    helper[key].totalCount += o.countIn;
  }
  return r;
}, []);

console.log(locationCounts);

This is what I’m looking for as output…

[
  {"location": "228", "totalCount": 18},
  {"location": "229", "totalCount": 31},
  {"location": "230", "totalCount": 7}
];
Tags:

2

Answers


  1. 0

    I added an Object.entries to convert to object array

    let locationCounts = Object.entries(
  locationPeriodCounts
  .reduce(function(acc, {location,countIn}) {
    acc[location] ??= 0; // create if it does not exist
    acc[location] += countIn; // always add to the total
    return acc;
  }, {})
).map(([location, totalCount]) => ({location,  totalCount }));

console.log(locationCounts)
    <script>
    let locationPeriodCounts = [
      {"period": "2023-10-21", "location": "228", "countIn": 6},
      {"period": "2023-10-22", "location": "228", "countIn": 8},
      {"period": "2023-10-23", "location": "228", "countIn": 3},
      {"period": "2023-10-24", "location": "228", "countIn": 1},
      {"period": "2023-10-21", "location": "229", "countIn": 5},
      {"period": "2023-10-22", "location": "229", "countIn": 18},
      {"period": "2023-10-23", "location": "229", "countIn": 8},
      {"period": "2023-10-24", "location": "230", "countIn": 3},
      {"period": "2023-10-25", "location": "230", "countIn": 4}
    ];

</script>
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  2. 0

    I found @mplungjan‘s response much more smart and compact than mine.
    This, however, is what I usually do:

    let locationPeriodCounts = [
    { "period": "2023-10-21", "location": "228", "countIn": 6 },
    { "period": "2023-10-22", "location": "228", "countIn": 8 },
    { "period": "2023-10-23", "location": "228", "countIn": 3 },
    { "period": "2023-10-24", "location": "228", "countIn": 1 },
    { "period": "2023-10-21", "location": "229", "countIn": 5 },
    { "period": "2023-10-22", "location": "229", "countIn": 18 },
    { "period": "2023-10-23", "location": "229", "countIn": 8 },
    { "period": "2023-10-24", "location": "230", "countIn": 3 },
    { "period": "2023-10-25", "location": "230", "countIn": 4 }
]

let locationCounts = locationPeriodCounts.reduce((acc, { location, countIn }) => {

    let currLocationCount = acc.find(count => count.location === location)

    if (currLocationCount) {
        currLocationCount.totalCount += countIn
    } else {
        acc.push({
            location: location,
            totalCount: countIn
        })
    }

    return acc;
}, []);

//If you also need sorting
locationCounts.sort((a, b) => {
    //return a.totalCount - b.totalCount // asc
    return b.totalCount - a.totalCount //desc
})

console.log(locationCounts);

    And this is the output:

    [
   {location: "229", totalCount: 31},
   {location: "228", totalCount: 18},
   {location: "230", totalCount: 7}
]
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