reduce & sum array of objects entries

Question

From the following array:

var arr = [{ "Year": 2019, "Title": "Sample1", "Sum": 1020000.0, "Budget":0}, 
{  "Year": 2019,  "Title": "Sample2",  "Sum": 2546658.0,  "Budget":100},
{  "Year": 2019,  "Title": "Sample3",  "Sum": 1020000.0,  "Budget":1000},
{  "Year": 2020,  "Title": "Sample1",  "Sum": 3472000.0,  "Budget":100}, 
{  "Year": 2020,  "Title": "Sample2",  "Sum": 1020000.0,  "Budget":10},
{  "Year": 2020,  "Title": "Sample3",  "Sum": 2452000.0,  "Budget":50},
{  "Year": 2021,  "Title": "Sample1",  "Sum": 1000.0,  "Budget":100}, 
{  "Year": 2021,  "Title": "Sample2",  "Sum": 119000.0,  "Budget":10},
{  "Year": 2021,  "Title": "Sample3",  "Sum": 234000.0,  "Budget":50}]
]

I need to change this into a single year per row, were the value of each "Title" has an entry with its "Sum" value and the Budget values should be aggregated together ie.

[{ "Year": 2019, "Sample1": 1020000.0, "Sample2":2546658.0, "Sample3":1020000.0 , "Budget":1100},{  etc]

My platform does not support ES6, through answers from an earlier post I have used .reduce as follows to get most of the way:

var res = arr.reduce(function(acc, curr) {
  acc[curr.Year] = acc[curr.Year];
  acc[curr.Year] = acc[curr.Year] || { Year: curr.Year } ;
  acc[curr.Year][curr.Title] = curr.Sum;
  return acc;

res = Object.keys(res).map(function(key) {
return res[key];
    });

This produces:

[{ "Year": 2019, "Sample1": 1020000.0, "Sample2":2546658.0, "Sample3":1020000.0 }, 
{ "Year": 2020, "Sample2": 3472000.0, "Sample2":1020000.0, "Sample3":2452000.0}, 
{ "Year": 2021, "Sample3": 1000.0, "Sample2":119000.0, "Sample3":234000.0}]

But I cannot find a way to also sum the Budget figures together and add it to the same entry. I suspect I need to perform a separate reduce function on a duplicate array and push the result into the res array using the forEach loop with Year as the key. Can anyone see a way of doing this in the same reduce function?

CertainPerformance · Accepted Answer · 2019-06-04 01:27:31Z

When initializing a Year object in the reduce callback, also initialize a Budget property to 0. Then, on each iteration for that year, add to the budget property in addition to setting the Sample property:

var arr = [{ "Year": 2019, "Title": "Sample1", "Sum": 1020000.0, "Budget":0}, 
  {  "Year": 2019,  "Title": "Sample2",  "Sum": 2546658.0,  "Budget":100},
  {  "Year": 2019,  "Title": "Sample3",  "Sum": 1020000.0,  "Budget":1000},
  {  "Year": 2020,  "Title": "Sample1",  "Sum": 3472000.0,  "Budget":100}, 
  {  "Year": 2020,  "Title": "Sample2",  "Sum": 1020000.0,  "Budget":10},
  {  "Year": 2020,  "Title": "Sample3",  "Sum": 2452000.0,  "Budget":50},
  {  "Year": 2021,  "Title": "Sample1",  "Sum": 1000.0,  "Budget":100}, 
  {  "Year": 2021,  "Title": "Sample2",  "Sum": 119000.0,  "Budget":10},
  {  "Year": 2021,  "Title": "Sample3",  "Sum": 234000.0,  "Budget":50}
]
var res = arr.reduce(function(acc, curr) {
  acc[curr.Year] = acc[curr.Year] || { Year: curr.Year, Budget: 0 } ;
  //                                                    ^^^^^^^^^
  acc[curr.Year][curr.Title] = curr.Sum;
  acc[curr.Year].Budget += curr.Budget;
  // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  return acc;
}, {});
var output = Object.keys(res).map(function(key) {
  return res[key];
});
console.log(output);

Note that the line in your original code

acc[curr.Year] = acc[curr.Year];

doesn't accomplish anything at all - you may omit it entirely.

You could consider using Babel and polyfills, allowing you to write code in the latest and greatest version of the language, while preserving compatibility for obsolete browsers, in which case, the code could be prettified to:

var arr=[{"Year":2019,"Title":"Sample1","Sum":1020000.0,"Budget":0},{"Year":2019,"Title":"Sample2","Sum":2546658.0,"Budget":100},{"Year":2019,"Title":"Sample3","Sum":1020000.0,"Budget":1000},{"Year":2020,"Title":"Sample1","Sum":3472000.0,"Budget":100},{"Year":2020,"Title":"Sample2","Sum":1020000.0,"Budget":10},{"Year":2020,"Title":"Sample3","Sum":2452000.0,"Budget":50},{"Year":2021,"Title":"Sample1","Sum":1000.0,"Budget":100},{"Year":2021,"Title":"Sample2","Sum":119000.0,"Budget":10},{"Year":2021,"Title":"Sample3","Sum":234000.0,"Budget":50}]

const output = Object.values(arr.reduce((a, { Year, Title, Sum, Budget }) => {
  a[Year] = a[Year] || { Year, Budget: 0 };
  a[Year][Title] = Sum;
  a[Year].Budget += Budget;
  return a;
}, {}));
console.log(output);

(using shimmed ES6) ...or even Object.values(arr.reduce((a, { Year, Title, Sum, Budget }) => ({...a, [Year] : {...(a[Year] || {}), Year, [Title]: Sum, Budget: (a[Year] || {Budget: 0}).Budget + Budget}}), {}))

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