How do I take an array of objects from state and push a specific value of the objects into a new array

Question

I am receiving a json object from my API and I am pushing it into my state. That object has a field called "grades". That grades is an array of objects that have a field called "mark".

I would like to take the "mark" from all the objects and push it into a new array / state in order to calculate the average "mark".

Here's my code that includes comments on what I would like to do:

const [trainee, setTrainee] = useState ({})

// state declaration for the trainee.grades.mark array
const [mark, setMark] = useState ([])

useEffect(() => {
    fetch(`/api/trainees/${match.params.id}`)
    .then(response => response.json())
    .then(json => setTrainee(json))
// something here that pushes all trainee.grades.mark into mark through setMark
}, [])


// function that takes the state Mark array and calculates the average of it
function arrayAverage(arr){
  //Find the sum
  var sum = 0;
  for(var i in arr) {
      sum += arr[i];
  }
  //Get the length of the array
  var numbersCnt = arr.length;
  //Return the average / mean.
  return (sum / numbersCnt);
}

// end result by calling the average calculator function and passing mark as an array in it

 <Typography className={classes.title} color="textSecondary" gutterBottom>
          AVERAGE GRADE
        </Typography>
        <Typography variant="body2" component="p">
            {arrayAverage(mark)}
        </Typography>
        <Divider />

Here's the trainee object

_id: String
fullName: String
age: Number
author: {}
department: String
endDate: Date
startDate: Date
status: String
grades: [{  ._id: String
            title: String
            text: String
            mark: Number
            }]

Answer 1

You could make marks array like this

useEffect(() => {
  fetch(`/api/trainees/${match.params.id}`)
  .then(response => response.json())
  .then(json => {
    setTrainee(json);
    setMark(json.grades.map(grade => grade.mark));
  })
}, [])

and to calculate average

function arrayAverage(arr){
  //Find the sum
  const sum = arr.reduce((accumulator, mark) => (accumulator + mark));
  return sum / arr.length;
}

Answer 2

There is no other way but to traverse through the entire array of grades. You can either use Array.map or a for loop to traverse the grades array. if the parsed object is like

let response = {
  grades: [
    {
       mark: value
       ...
    },
    ...
  ]
}

You can use

let marks = response.grades.map(grade => {
   return grade.mark;
})

Now marks will contain a list of mark values. You can call setMarks(marks) to update the state.