函数返回未定义的nodejs

Question

Here I'm creating an array that have property 1, then I'm trying to filter the results with only the highest number and then output it when calling the function, but it returns undefined. I'm trying to find to figure out what I'm doing wrong here. Thanks

function calcDifference() {
  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()); // returns undefined

Answer 1

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()); 

Answer 2

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
});