Javascript arrays: group by unique values and sum-up amount per value

Question

I'm trying to build my own app, and I'm stuck at one issue that I cannot solve. My app deals with mortgages. I have several mortgages running in parallel, and I just need to sum-up total payments per period.

Let's say that we have the following arrays of objects (i.e. mortgages):

[ {uptomonth:84 , payment:150} ] 
[ {uptomonth:120 , payment:200} ] 
[ {uptomonth:120 , payment:100} , {uptomonth:180 , payment:250} , {uptomonth:300 , payment:500} ]

How to read this (example for 1st line): "up to month number 84, I pay $150 per month".

I want to combine arrays into one array (e.g. using array.concat...), then sort objects by "uptomonth" in order to obtain a result array like that:

[ {uptomonth:84,payment:1200} , {uptomonth:120,payment:1050} , {uptomonth:180,payment:750} , {uptomonth:300,payment:500} ]

The most difficult for me is to group by "uptomonth" (since there are duplicates of this value), and get the total payments per "uptomonth"...

Any idea of how to do that? Thanks a lot!

[SOI} is not a code writing service: please show what you have tried so far? (Hint: solve the group by on its own.) — Richard, Aug 30 '16 at 08:52
So for example umonth: 120 payment value should be 300 (100+200)? — Maciej Sikora, Aug 30 '16 at 08:57

Nenad Vracar · Answer 1 · 2016-08-30T15:42:59.197

You can do this task like this:

Flatten the array to array with objects only
Sort that new array
Calculate total payment
Group by uptomonth and calculate payment for each one

var data = [[ {uptomonth:84 , payment:150} ],[ {uptomonth:120 , payment:200} ],[ {uptomonth:120 , payment:100} , {uptomonth:180 , payment:250} , {uptomonth:300 , payment:500} ] ];

var finalResult = [];

//Flatten the array
var newArray = data.reduce(function(r, a) {
  a.forEach(function(o) {r.push(o)});
  return r;
}, [])

//Sort the array
newArray.sort(function(a, b) {
  return a.uptomonth - b.uptomonth;
});

//Get total payment
var total = newArray.reduce(function(r, a) {
  return r = r + a.payment;
}, 0)

//Group by uptomonth and calculate payment for each one
newArray.forEach(function(o) {
  if (!this.payment) this.payment = total;
  if (!this[o.uptomonth]) {
    this[o.uptomonth] = {uptomonth: o.uptomonth, payment: this.payment}
    finalResult.push(this[o.uptomonth]);
  }
  this.payment -= o.payment;
}, {});

console.log(finalResult)

Tarek Salah uddin Mahmud · Answer 2 · 2016-08-30T11:35:56.080

Try this

var data = [[ {uptomonth:84 , payment:150} ],[ {uptomonth:120 , payment:200} ],[ {uptomonth:120 , payment:100} , {uptomonth:180 , payment:250} , {uptomonth:300 , payment:500} ] ];

var newData = {};

data.forEach(function(array)
         { array.forEach(function(node)
                     {
                         if(!newData[node.uptomonth]){newData[node.uptomonth]=0};
                         newData[node.uptomonth] += node.payment;
                     });
         });

var ans = [];

Object.keys(newData).forEach(function(key){
    var abc = {};
    abc.uptomonth = key;
    abc.payment=newData[key];
    ans.push(abc);
});

ans.sort(function(a,b){return parseInt(a.uptomonth)>parseInt(b.uptomonth)});

the variable ans is your desired array.

score 0 · Answer 3 · answered Aug 30 '16 at 11:55

I'd recommend you to use _.flatten method from lodash.

var data = [[ {uptomonth:84 , payment:150} ], 
            [ {uptomonth:120 , payment:200} ] ,
            [ {uptomonth:120 , payment:100} , 
              {uptomonth:180 , payment:250} , 
              {uptomonth:300 , payment:500} ]]

_
 .chain(data)
 .flatten()
 .reduce((acc, obj) => {
   var month = obj.uptomonth
   var payment = obj.payment

   if (acc[month]) acc[month] += payment
   else acc[month] = payment

   return acc
 }, {})
 .value()

Javascript arrays: group by unique values and sum-up amount per value

3 Answers3