Utilizing asynchronous calls within a forEach loop for executing Firebase operations

Question

Utilizing asynchronous calls within a forEach loop for executing Firebase operations

My dilemma lies in the inability to successfully implement this code using Firestore (although I'm not entirely sure if this is relevant).

The specific code snippet is as follows:

prestamoItems() {
      var myarray = [];
      var myobject = {};

      //Beginning of first async method (executes properly)

      fb.prestamosCollection
        .orderBy("fechaPrestamo", "desc")
        .get()
        .then(val => {
          if (!val.empty) {

            //Here begins the forEach loop

            val.docs.forEach(doc => {
              myobject = doc.data();
              myobject.id = doc.id;
              console.log("The doc id is " +myobject.id)

              //Second asynchronous call inside the forEach loop, but it doesn't wait for completion 
              //before moving on to the next step

              fb.equiposCollection.doc(myobject.id).get().then(eqp => {
                console.log("The doc id from the other collection is " +eqp.id)
              })

              myarray.push(myobject)
              console.log("myobject pushed to myarray")
            });


          }
        });
    }

I want to highlight that I have an asynchronous method being called within a forEach loop nested within another asynchronous method. Despite various iterations of the code structure, the console logs consistently display the following output:

11:13:14.999 Prestamos.vue?18d2:71 The doc id is 1yTCUKwBvlopXX2suvVu
11:13:14.999 Prestamos.vue?18d2:78 myobject pushed to myarray
11:13:15.000 Prestamos.vue?18d2:71 The doc id is Z5TE15Fj3HFrn1zvceGe
11:13:15.000 Prestamos.vue?18d2:78 myobject pushed to myarray
11:13:15.000 Prestamos.vue?18d2:71 The doc id is JNN9aN65XE1tUTmlzkoJ
11:13:15.000 Prestamos.vue?18d2:78 myobject pushed to myarray
11:13:15.000 Prestamos.vue?18d2:71 The doc id is NF2hHCpM8leZezHbmnJx
11:13:15.001 Prestamos.vue?18d2:78 myobject pushed to myarray
11:13:15.364 Prestamos.vue?18d2:74 The doc id from the other collection is 1yTCUKwBvlopXX2suvVu
11:13:15.368 Prestamos.vue?18d2:74 The doc id from the other collection is Z5TE15Fj3HFrn1zvceGe
11:13:15.374 Prestamos.vue?18d2:74 The doc id from the other collection is JNN9aN65XE1tUTmlzkoJ
11:13:15.379 Prestamos.vue?18d2:74 The doc id from the other collection is NF2hHCpM8leZezHbmnJx

Hence, the behavior demonstrates that the forEach loop does not pause for the inner asynchronous function to complete, which aligns with expectations.

In light of this, my question is how do I ensure the loop waits for the internal call to finish before appending the object to the array? Any insights would be greatly appreciated.

javascript vue.js asynchronous vuejs2

Answer 1

Answer №1

To incorporate code that relies on previous outcomes into then() callbacks, you can either nest the code or encapsulate the loop (since forEach doesn't support async operations) in an async block to utilize await within it. For example:

fb.prestamosCollection
  .orderBy("fechaPrestamo", "desc")
    .get()
    .then(val => {
      if (!val.empty) {
        // Enclose the loop within an async function call immediately invoked function expression (iife) to enable usage of await inside
        (async () => {
          for (var i = 0; i < val.docs.length; i++) {
            const doc = val.docs[i];
            myobject = doc.data();
            myobject.id = doc.id;
            // Execution will now be synchronous
            let eqp = await fb.equiposCollection.doc(myobject.id).get();
            console.log(eqp.id);
            myarray.push(myobject)
          }
        })();
      }
    });

Answer 2

To incorporate code that relies on previous outcomes into then() callbacks, you can either nest the code or encapsulate the loop (since forEach doesn't support async operations) in an async block to utilize await within it. For example:

fb.prestamosCollection
  .orderBy("fechaPrestamo", "desc")
    .get()
    .then(val => {
      if (!val.empty) {
        // Enclose the loop within an async function call immediately invoked function expression (iife) to enable usage of await inside
        (async () => {
          for (var i = 0; i < val.docs.length; i++) {
            const doc = val.docs[i];
            myobject = doc.data();
            myobject.id = doc.id;
            // Execution will now be synchronous
            let eqp = await fb.equiposCollection.doc(myobject.id).get();
            console.log(eqp.id);
            myarray.push(myobject)
          }
        })();
      }
    });

Answer 3

Answer №2

The underlying issue stems from attempting to convert an asynchronous operation (waiting for Firestore to retrieve values) into a synchronous one. Accomplishing this in a meaningful manner within JavaScript poses significant challenges!

To resolve this, you must populate your array within the .then() callback and return the promise as the outcome of the function. Any invocation of your prestamoItems() function will also necessitate using .then() callbacks to access the underlying myarray value:

const _ = {
  async prestamoItems() {
    const val = await fb.prestamosCollection.orderBy("fechaPrestamo", "desc").get();
    if (val.empty) {
      return myarray
    }

    // Promise.all() aggregates multiple promises and returns their results upon completion.
    return await Promise.all(
      // Array.prototype.map() executes a function on each item in an array and gathers the returned values in a new array.
      // This is akin to building a new array by using push(), but it's more readable!
      val.docs.map(async doc => {
        const myobject = doc.data();
        const eqp = await fp.equiposCollection.doc(myobject.id).get()
        // Presumably, further actions involving eqp are intended here
        return myobject
      })
    );
  }
}

The provided code snippet employs Array.prototype.map(), rendering myarray superfluous.

A user would need to utilize this code as follows:

_.prestamoItems().then((myarray) => {
   ...
})

Promises signify that a value may become available at some future point. Accordingly, all interactions with a promise should assume the value isn't immediately accessible. The recommended approach involves employing async/await and ensuring the consistent usage of promise objects.

Answer 4

The underlying issue stems from attempting to convert an asynchronous operation (waiting for Firestore to retrieve values) into a synchronous one. Accomplishing this in a meaningful manner within JavaScript poses significant challenges!

To resolve this, you must populate your array within the .then() callback and return the promise as the outcome of the function. Any invocation of your prestamoItems() function will also necessitate using .then() callbacks to access the underlying myarray value:

const _ = {
  async prestamoItems() {
    const val = await fb.prestamosCollection.orderBy("fechaPrestamo", "desc").get();
    if (val.empty) {
      return myarray
    }

    // Promise.all() aggregates multiple promises and returns their results upon completion.
    return await Promise.all(
      // Array.prototype.map() executes a function on each item in an array and gathers the returned values in a new array.
      // This is akin to building a new array by using push(), but it's more readable!
      val.docs.map(async doc => {
        const myobject = doc.data();
        const eqp = await fp.equiposCollection.doc(myobject.id).get()
        // Presumably, further actions involving eqp are intended here
        return myobject
      })
    );
  }
}

The provided code snippet employs Array.prototype.map(), rendering myarray superfluous.

A user would need to utilize this code as follows:

_.prestamoItems().then((myarray) => {
   ...
})

Promises signify that a value may become available at some future point. Accordingly, all interactions with a promise should assume the value isn't immediately accessible. The recommended approach involves employing async/await and ensuring the consistent usage of promise objects.

Answer 5

Answer №3

To properly execute the process, ensure to shift the push operation internally:

fb.equiposCollection.doc(myobject.id).get().then(eqp => {
   console.log("The doc id from the other collection is " +eqp.id)

   myarray.push(myobject)
   console.log("myobject successfully added to myarray")
})

Answer 6

To properly execute the process, ensure to shift the push operation internally:

fb.equiposCollection.doc(myobject.id).get().then(eqp => {
   console.log("The doc id from the other collection is " +eqp.id)

   myarray.push(myobject)
   console.log("myobject successfully added to myarray")
})

Utilizing asynchronous calls within a forEach loop for executing Firebase operations

Answer №1

Answer №2

Answer №3

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