for循环中的if/else语句

Question

I have a for loop that gathers locations from the Firebase Database. From this data I also calculate a distance between the users phone and the location it self.

My app now breaks if the userData is not found, so I would like to create a failsafe for if the user location isn't there. However I am not able to get this to work...both the console.log entries from the if AND else statement aren't fired. I also don't see the Does this work outside comment. It's late for me so maybe I'm missing something obvious?

 for (const key in resData) { const reduxUserLocation = getState().locationActions.userLocation if (reduxUserLocation === null) { const calculateDistance = getDistance( { latitude: resData[key].location[0].lat, longitude: resData[key].location[0].lng }, { latitude: reduxUserLocation.lat, longitude: reduxUserLocation.lng } ) console.log('distance?', calculateDistance) loadLocations.push( new ReportedLocations( key, resData[key].ownerId, resData[key].location, resData[key].description, resData[key].streetname, resData[key].placename, resData[key].date, calculateDistance ) ); } else { console.log("user location isn't found") const calculateDistance = 0 loadLocations.push( new ReportedLocations( key, resData[key].ownerId, resData[key].location, resData[key].description, resData[key].streetname, resData[key].placename, resData[key].date, calculateDistance ) ); } console.log('Does this work outside the if/else statement?') }

Answer 1

It's hard to debug without seeing the code you use for the firestore query. If I had to guess I would look into checking for empty documents/queries using calls to the SDK as shown in the documentation . I customized the example code in the docs based on your code.

Here's some code for a single document read.

let resData;
const doc = await cityRef.get();
if (!doc.exists) {
  console.log('No such document!');
  resData = null
} else {
  resData = doc.data();
}

and a collection

let resDataArr = []
const snapshot = await citiesRef.where('capital', '==', true).get();
if (snapshot.empty) {
  console.log('No matching documents.');
  resDataArr = [null]
} else {
  snapshot.forEach(doc=> resDataArr.push(doc.data()) );
}

This may change if you're using a library. I didn't test this code so there may be a syntax error or two.

Answer 2

So it appears you are trying to retrieve a lat/long value from reduxUserLocation when it is null

if (reduxUserLocation === null) {
    const calculateDistance = getDistance(
        { latitude: resData[key].location[0].lat, longitude: resData[key].location[0].lng },
        { latitude: reduxUserLocation.lat, longitude: reduxUserLocation.lng } // THIS LINE
    )

Obviously this is not possible and it would throw an error. Therefor, you are not seeing any log lines below it.

Furthermore, I believe your if-statement is incorrect. It seems like you want to calculate the distance if possible and else it should be 0. Your if-statement does the reverse.

Besides that, there is a catch when using a for-in loop. Best to always check if hasOwnProperty

If you only want to consider properties attached to the object itself, and not its prototypes, use getOwnPropertyNames() or perform a hasOwnProperty() check (propertyIsEnumerable() can also be used). Alternatively, if you know there won't be any outside code interference, you can extend built-in prototypes with a check method. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in

Here is, what I believe should be the correct code

for (const key in resData) {
  // Check if key is a property, if not, skip this iteration
  if (!resData.hasOwnProperty(key)) {
    continue;
  }

  const reduxUserLocation = getState().locationActions.userLocation;

  let calculateDistance = 0; // Set default distances to 0

  // If location is present, use that to calculate the distance and overwrite the variable above
  if (reduxUserLocation){
    calculateDistance = getDistance(
      {latitude: resData[key].location[0].lat, longitude: resData[key].location[0].lng},
      {latitude: reduxUserLocation.lat, longitude: reduxUserLocation.lng}
    );
  }
 
  // Moved push to array after the if-statement -> DRY (Don't Repeat Yourself)
  loadLocations.push(
    new ReportedLocations(
      key,
      resData[key].ownerId,
      resData[key].location,
      resData[key].description,
      resData[key].streetname,
      resData[key].placename,
      resData[key].date,
      calculateDistance
    )
  );
}