Loop through data to pull array of firebase values

Question

I have a an array of a number of N paths to retrieve data from a different location in firebase database.

searchPaths = ['locations/date1/imageID', 'locations/date2/imageID2', locations/date3/imageID3, ...]

now, I want to loop through each search path and pull a value from it to save an array of image URL's.

    const searchPaths = ['locations/date1/imageID', 'locations/date2/imageID2']
    const imageURLs = []

     for(var Obj in searchPaths) 
    {
        const path = Obj
        admin.database().ref(path).once('value').then(snapshot => 
        { 
        const URL = snapshot.val().fileURL;
        imageURLs.push(URL);
        console.log('ImageURL: ' + URL );
        })
    // here is where it gets sour
    }.then(() => {
        console.log("All image URL's" + imageURLs")
    }

So, my question is, how do I return a promise when we have now pulled the data we need from every ref? is there a Promise.all type? where does it go?

Answer 1

You can use the for-loop to create an array of promises, then use Promise.all , crude I know but it should work.

const searchPaths = ['locations/date1/imageID', 'locations/date2/imageID2']
const imageURLs = []
var promises = [];
 for(var Obj in searchPaths) 
{
    promises.push(new Promise(function(resolve, reject) {
      const path = Obj
      admin.database().ref(path).once('value').then(snapshot => 
      { 
      const URL = snapshot.val().fileURL;
      imageURLs.push(URL);
      console.log('ImageURL: ' + URL );

      //resolve the promise after pushing imageURL
      resolve();
      })
    }));
}

//when all of them are done:
Promise.all(promises)
.then(function(results) {
  //code when done...
})

Answer 2

The other answer here is going through too much trouble to collect promises. It's easier to just push the return value of once() into the promises array rather than creating a new promise each time.

const searchPaths = ['locations/date1/imageID', 'locations/date2/imageID2']
const imageURLs = []
const promises = []   // collect promises here

searchPaths.forEach(path => {
    promises.push(admin.database().ref(path).once('value'))
})

Promise.all(promises).then(results => {
    results.forEach(snapshot => {
        const URL = snapshot.val().fileURL;
        imageURLs.push(URL);
        console.log('ImageURL: ' + URL );
    }
})

The then callback on the promise returned from Promise.all() will be an array of all the snapshots from the queries pushed into the promises array.