How to aggregate objects with dates as keys in javascript

Question

I have an object with keys as dates in the following format "yyyy-mm-dd" (ex: 2020-08-14) the value of each key is an array of objects with two attributes, name and value.

I need to find a way to get the sum of the keyvalue grouped by name accros N days.

Here is an example to better understand, the original object have 4 days of data, with one day having an empty array:

{
  "2020-10-15":[
    {"name":"France","value":20},
    {"name":"United States","value":20},
    {"name":"Italy","value":5},
   ],
 "2020-10-16":[
    {"name":"Germany","value":10},
    {"name":"United States","value":5},
    {"name":"France","value":10}
   ],
  "2020-10-17":[],
   "2020-10-18":[
    {"name":"UK","value":10},
    {"name":"Italy","value":10},
   ]

 }

For example if we wish to group this data by 4 days we will get the following:

[{"name": "France", "value": 30},
 {"name": "United States", "value": 25},
 {"name": "Italy", "value": 15},
 {"name": "Germany", "value": 10},
 {"name": "UK", "value": 10}]

This is the sum of all objects with same value for name across 4 days. I have absolutly no idea how to achieve this, I know can use map to iterate the object keys and do some processing through moment js but I don't know how to achieve this.

start the way you do know how to do it: don't try to use clever .map or .entries etc, just create an empty array, start a for loop, and have your code do things the more verbose, much at least understandable-to-yourself way. — Mike 'Pomax' Kamermans
– Mike 'Pomax' Kamermans, Commented Nov 17, 2020 at 19:24

Md Sabbir Alam · Accepted Answer · 2020-11-17 19:32:57Z

2

You can do the following using reduce, forEach and Object.keys method,

const value = {
  "2020-10-15":[
    {"name":"France","value":20},
    {"name":"United States","value":20},
    {"name":"Italy","value":5},
   ],
 "2020-10-16":[
    {"name":"Germany","value":10},
    {"name":"United States","value":5},
    {"name":"France","value":10}
   ],
  "2020-10-17":[],
   "2020-10-18":[
    {"name":"UK","value":10},
    {"name":"Italy","value":10},
   ]

 }

let res = Object.keys(value).reduce((prev, curr) => {
   value[curr].forEach(item => {
      const idx = prev.findIndex(pItem => pItem.name === item.name);
      if(idx > -1) {
         const newObj = {...prev[idx], value: prev[idx].value + item.value};
         prev[idx] = newObj;
         return ;
      }
      prev.push(item);
   })
   return prev;
}, []);
console.log(res);

answered Nov 17, 2020 at 19:32

Md Sabbir Alam

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2 Comments

Souames Over a year ago

This code works perfectly thank you but I couldn't really understand it, could you add some explanation to your answer please ?

Md Sabbir Alam Over a year ago

I will add an explanation tomorrow. In the mean time you can read the links I have provided. It should be sufficient to get some idea whats happenning here.

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