using reduce to sum values in array of objects gives error

October 13, 2022

I want to sum up the thinking like the following code:

const bests = [
 {
   thinking: { _id: '6347d5b4edc3227f056cbc0c', sum: 64, num: 1 },
    listening: { _id: '6347d5b4edc3227f056cbc0d', sum: 129, num: 2 },
    speaking: { _id: '6347d5b4edc3227f056cbc0e', sum: 0, num: 2 },
    writing: { _id: '6347d5b4edc3227f056cbc0f', sum: 0, num: 0 },
    speed: { _id: '6347d5b4edc3227f056cbc10', sum: 187, num: 2 },
    overall: { _id: '6347d5b4edc3227f056cbc11', sum: 129, num: 2 }
  },
 {
     thinking: { _id: '6347e46665609042876fc4a6', sum: 100, num: 1 },
    listening: { _id: '6347e46665609042876fc4a7', sum: 100, num: 1 },
     speaking: { _id: '6347e46665609042876fc4a8', sum: 100, num: 1 },
     writing: { _id: '6347e46665609042876fc4a9', sum: 0, num: 0 },
     speed: { _id: '6347e46665609042876fc4aa', sum: 88, num: 1 },
     overall: { _id: '6347e46665609042876fc4ab', sum: 100, num: 1 }
   },
   {
     thinking: { _id: '6347e54f65609042876fc4af', sum: 10, num: 10 },
    listening: { _id: '6347e54f65609042876fc4b0', sum: 65, num: 1 },
     speaking: { _id: '6347e54f65609042876fc4b1', sum: 0, num: 1 },
    writing: { _id: '6347e54f65609042876fc4b2', sum: 0, num: 0 },
    speed: { _id: '6347e54f65609042876fc4b3', sum: 95, num: 1 },
    overall: { _id: '6347e54f65609042876fc4b4', sum: 65, num: 1 }
  }
 ]
 
 
 

const sum = bests.reduce((a, b) => a['thinking'].sum + b['thinking'].sum);
console.log('sum::', sum);

But as you see I get the wired error!
How can I fix this?

>Solution :

The following code give you the expected results.

const bests = [
 {
   thinking: { _id: '6347d5b4edc3227f056cbc0c', sum: 64, num: 1 },
    listening: { _id: '6347d5b4edc3227f056cbc0d', sum: 129, num: 2 },
    speaking: { _id: '6347d5b4edc3227f056cbc0e', sum: 0, num: 2 },
    writing: { _id: '6347d5b4edc3227f056cbc0f', sum: 0, num: 0 },
    speed: { _id: '6347d5b4edc3227f056cbc10', sum: 187, num: 2 },
    overall: { _id: '6347d5b4edc3227f056cbc11', sum: 129, num: 2 }
  },
 {
     thinking: { _id: '6347e46665609042876fc4a6', sum: 100, num: 1 },
    listening: { _id: '6347e46665609042876fc4a7', sum: 100, num: 1 },
     speaking: { _id: '6347e46665609042876fc4a8', sum: 100, num: 1 },
     writing: { _id: '6347e46665609042876fc4a9', sum: 0, num: 0 },
     speed: { _id: '6347e46665609042876fc4aa', sum: 88, num: 1 },
     overall: { _id: '6347e46665609042876fc4ab', sum: 100, num: 1 }
   },
   {
     thinking: { _id: '6347e54f65609042876fc4af', sum: 10, num: 10 },
    listening: { _id: '6347e54f65609042876fc4b0', sum: 65, num: 1 },
     speaking: { _id: '6347e54f65609042876fc4b1', sum: 0, num: 1 },
    writing: { _id: '6347e54f65609042876fc4b2', sum: 0, num: 0 },
    speed: { _id: '6347e54f65609042876fc4b3', sum: 95, num: 1 },
    overall: { _id: '6347e54f65609042876fc4b4', sum: 65, num: 1 }
  }
 ]
 
 // changed code block start

const sum = bests.reduce((a, b) => a + b['thinking'].sum,0);

 // changed code block end

console.log('sum::', sum);

Explanation : a is the sum of all values before adding current value. Therefore it is a number not an item of this list and calling a['thinking'].sum cause error because a is a number therefore a['thinking'] is undefined. b is the current item of list. Also you have to pass the initial value which is 0.