Javascript: Merge Array of Objects, summing values with same key

Question

I already searched for my issue, however, i did not find something, that matches my needs. I want to merge (sum) multiple series of data into a single array. Any dataset that matches the key, shall be summed in the resulting array.

Please see sample data and expected result:

var power1 = [
  {
    "time": 10,
    "power": 100
  },
  {
    "time": 20,
    "type": 200
  },
  {
    "time": 30,
    "type": 300
  }
]
var power2 = [
  {
    "time": 20,
    "type": 200
  },
  {
    "time": 30,
    "type": 300
  },
  {
    "time": 40,
    "type": 400
  }
]    
var result = [
  {
    "time": 10,
    "type": 100
  },
  {
    "time": 20,
    "type": 400
  },
  {
    "time": 30,
    "type": 600
  },
  {
    "time": 40,
    "type": 400
  }
]

Since this should happen with thousands of items, it should be reasonable fast. Could a Map as intermediate help here?

Thanks in advance

After you concat 2 arrays, you have all kind of answers here — Nikola Pavicevic
– Nikola Pavicevic, Commented Aug 3, 2021 at 22:37
Unfortunately, this did not work for me. But i am using Node Red. Maybe there are limitations regarding JavaScript. However, the hint from @NikolaPavicevic helped me. The overall solution now looks different: since my data is coming from an InfluxDB, i can do all that math in a query. Thanks a lot! — mincequi
– mincequi, Commented Aug 12, 2021 at 10:40

Unmitigated · Accepted Answer · 2021-08-03 22:13:25Z

1

You can concat the two arrays and then perform a reduce operation over it with an object to store the values for each key.

var power1=[{time:10,type:100},{time:20,type:200},{time:30,type:300}],power2=[{time:20,type:200},{time:30,type:300},{time:40,type:400}];
const res = Object.values(
  power1.concat(power2).reduce((acc, {type, time})=>{
    (acc[time] ??= {time, type: 0}).type += type;
    return acc;
  }, {})
);
console.log(res);

answered Aug 3, 2021 at 22:13

Unmitigated

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2 Comments

lonewarrior556 Over a year ago

what is this ??=

Ben Stephens Over a year ago

@lonewarrior556 it's logical nullish assignment: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

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