Function returns undefined nodejs

You haven't returned anything from that function.

You should add return before table.find

function calcDifference(){

      return table.find({"entries":1}) // select entries with "entries":1

        .toArray(function(err, res_spr) { // create an array of found elements 
 
              let pop = res_spr.pop(); // delete last element of the array

              let storeArraySpr = []; // new array to store property "number"

              for (let i = 0; i < res_spr.length; i++) { 

                storeArraySpr.push(res_spr[i].number); 

              }

              var resultSpr = Math.max(...storeArraySpr); // select highest entry

              return resultSpr.toFixed(8); // meant to output the result when calling the function
        });
    } 

    console.log(calcDifference()); 

If toArray does not return values in that callback function. You can assign a value and return it.

Note that this approach will work if toArray and find do not return promises

function calcDifference(){
      let result;
      table.find({"entries":1}) // select entries with "entries":1

        .toArray(function(err, res_spr) { // create an array of found elements 
 
              let pop = res_spr.pop(); // delete last element of the array

              let storeArraySpr = []; // new array to store property "number"

              for (let i = 0; i < res_spr.length; i++) { 

                storeArraySpr.push(res_spr[i].number); 

              }

              var resultSpr = Math.max(...storeArraySpr); // select highest entry

              result = resultSpr.toFixed(8); // meant to output the result when calling the function
        });

     return result; //return the final result from the callback function
    } 

    console.log(calcDifference()); 

The 3rd approach, if find and toArray are actually promises

async function calcDifference(){
      return await table.find({"entries":1}) // select entries with "entries":1

        .toArray(function(err, res_spr) { // create an array of found elements 
 
              let pop = res_spr.pop(); // delete last element of the array

              let storeArraySpr = []; // new array to store property "number"

              for (let i = 0; i < res_spr.length; i++) { 

                storeArraySpr.push(res_spr[i].number); 

              }

              var resultSpr = Math.max(...storeArraySpr); // select highest entry

              return resultSpr
        });
    } 

    console.log(await calcDifference()); 

If toArray does not return results, you can assign variable again, but the difference is now we have async/await

async function calcDifference(){
         let result;
         await table.find({"entries":1}) // select entries with "entries":1

        .toArray(function(err, res_spr) { // create an array of found elements 
 
              let pop = res_spr.pop(); // delete last element of the array

              let storeArraySpr = []; // new array to store property "number"

              for (let i = 0; i < res_spr.length; i++) { 

                storeArraySpr.push(res_spr[i].number); 

              }

              var resultSpr = Math.max(...storeArraySpr); // select highest entry

              result = resultSpr
        });
        return result
    } 

    console.log(await calcDifference()); 

Your main issue is that calcDifference() doesn't have a return value. To solve this you first need to change the way you call toArray(). Like Barman pointed out in the comments:

When toArray() is called with a callback function, it doesn't return anything. See mongodb.github.io/node-mongodb-native/4.5/classes/… – Barmar

When you look at the documentation you'll find that it does return a promise if you do not pass a callback function. Firstly we change calcDifference to an async function. Then we await table.find({ "entries": 1 }).toArray() instead of passing a callback. After that you can do your other stuff.

Do note that an async function always returns a promise, so to use the return value you either have to await the value, or use a then() construct if you are not in async context.

async function calcDifference() {
  const res_spr = await table.find({ "entries": 1 }).toArray(); // select entries with "entries":1
  
  let pop = res_spr.pop(); // delete last element of the array
  let storeArraySpr = []; // new array to store property "number"
  for (let i = 0; i < res_spr.length; i++) {
    storeArraySpr.push(res_spr[i].number);
  }

  var resultSpr = Math.max(...storeArraySpr); // select highest entry
  return resultSpr.toFixed(8); // meant to output the result when calling the function
}

// when in async context
console.log(await calcDifference());

// when not in async context
calcDifference().then(console.log);

// or if you need to execute multiple statements
calcDifference().then((result) => {
  console.log(result);
  // other stuff
});