javascript sum value by key from array of array of objects

August 31, 2022

I want to sum an object value by key.
I have an array of arrays of objects.

Here:
[ 1: https://stackoverflow.com/questions/48550691/how-to-sum-all-of-values-from-specific-key-in-object][1]

I found this:

var arr = [{ 'credit': 1, 'trash': null }, { 'credit': 2, 'trash': null}];
console.log(arr.reduce((total, obj) => obj.credit + total,0));
// 3

I have multiple arrays of objects, thus:

var arr2 = [ [{ 'credit': 1, 'trash': null }, { 'credit': 2, 'trash': null}],
            [{ 'credit': 3, 'trash': null }, { 'credit': 3, 'trash': null}]
          ];

for (var i=0; i<arr2.length; i++) {
    console.log( arr2[i].reduce((total, arr2[i]) => arr2[i].credit + total,0))  ;
}

When trying to access a specific array I get this error message from Google Chrome "Uncaught SyntaxError: Invalid destructuring assignment target".

The editor I am using, Sublime, highlights a syntax error.

I don’t know how to fix it. I have googled without success.

I want to get the total for each iteration of the loop.
In the example given, that would :
3
then
6

TIA.

>Solution :

Flatten the array, then .reduce to sum.

var arr2 = [ [{ 'credit': 1, 'trash': null }, { 'credit': 2, 'trash': null}],
            [{ 'credit': 3, 'trash': null }, { 'credit': 3, 'trash': null}]
          ];

console.log(arr2.flat().reduce((total, obj) => obj.credit + total,0));

If you don’t want to flatten, you’ll have to use a nested .reduce for the inner arrays too.

var arr2 = [ [{ 'credit': 1, 'trash': null }, { 'credit': 2, 'trash': null}],
            [{ 'credit': 3, 'trash': null }, { 'credit': 3, 'trash': null}]
          ];

console.log(
  arr2.reduce(
    (total, subarr) => total + subarr.reduce(
      (a, { credit }) => a + credit,
      0
    ),
    0
  )
);

If you don’t care about the full total but only about each subarray’s total, then just perform the simple .reduce for every element of the array.

var arr2 = [ [{ 'credit': 1, 'trash': null }, { 'credit': 2, 'trash': null}],
            [{ 'credit': 3, 'trash': null }, { 'credit': 3, 'trash': null}]
          ];

for (const subarr of arr2) {
  console.log(subarr.reduce((total, obj) => obj.credit + total,0));
}