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How To Return The Response From An Asynchronous Call? [Answered]

return the response from an asynchronous call

“How to return the response from an asynchronous call?” is one of the most asked/searched questions by JS users. 

Well, the concept of asynchronous programming is well known to JavaScript experts. However, there are still various beginners who do not have any idea regarding this. 

If we say asynchronous in simple words, then it is the programming by which it is possible to express the long waiting actions to get executed without interrupting the main programs. 

A JavaScript user can implement this programming style using the callback and other functions. Moreover, they can return the call using these functions.

Do you not have an idea how to do this? Again, don’t worry! Just follow through as we have explained all the ways –how to return the response from an asynchronous call?  

Moreover, for beginners, we have detailed what asynchronous programming actually is. So, let’s find it out.

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Synchronous & Asynchronous

Let’s first understand what is synchronous and asynchronous?

The asynchronous models enable JavaScript users to do multiple things at the same time. In simple language, when the user starts an action, the program keeps on running. 

cartoon about request sync and frozen program in javascript

Once the action gets finished, the program will inform and proceed for accessing the result. For instance: The data is read from the disk.

Let’s understand asynchronous and synchronous programming with an example.

Say, a particular program fetches two different resources from a network to join them to provide a single result.

In the above diagram, the thick line shows the time that a program spends to run normally. On the other hand, the thin line shows the waiting time for the particular network. 

Here, for the synchronous model: The network’s time taken is the timeline’s part for the particular thread of control. 

Also, for the asynchronous model: The network action creates a split conceptually within the timeline. And the program continuously keeps on running, action occurs parallelly also notifying the program about the final status of the program.

Now, move to the solution for how to return the response from an asynchronous call.

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How to return the response from an asynchronous call?

Node.js Async Await Tutorial – With Asynchronous JavaScript Examples

Firstly, there are 2 different approaches for how to return the response from an asynchronous call. First, the callback function calls when the function is ready to get executed. And, the second one is a promise that returns the promise. So, let’s check both one by one.

Using callback function:

One of the approaches to asynchronous calls is the callback function. Here, the action starts, and when it gets completed, the callback function gets called along with the desired result. 

Or simply we can say that the callbacks() are useful for executing a function just after the other functions return.

For example, the setTimeout function waits for the provided milliseconds and then calls the desired function accordingly.

Output

100

Explanation

Here, the setTimeout function is called along with the callback (‘a’). So, this will execute after 1000 milliseconds. Whenever the function gets executed, it calls the passed ‘a’ callback along with the return value from the setTimeout callback.

Key point:
The setTimeout function belongs to the window object. And if we have setTimeout with 0ms, the function will not get invoked immediately. But, it gets placed in a queue to get invoked “as soon as possible” once the main event handler finishes running.

Using promise function:

Above all, the callback function has a drawback as it only represents the flow of the program. Also, it does not provide details about what the program does. 

So, how to sort it out? This is solved by the promise as it provides:

  • better error handling,
  • better control flow mechanism to asynchronous logic,
  • improved readability, 
  • reduce coupling.

A promise is one of the objects, which returns from the asynchronous function immediately. Furthermore, it allows the user to track that function’s state. 

Then, let’s see how it works:

Output

100

Explanation

Here, when the resolve function is called, it specifies that the asynchronous calls have been completed. To perform it, just wrap the function with the promise. Then, resolve the promise and pass it any callback for using the return value.

Key Point: The promise has three states:
pending: (initial) neither rejected nor fulfilled. rejected: operation gets failed. fulfilled: operation gets completed successfully

A short discussion

How do you handle asynchronous responses?

As discussed above, we have explained that asynchronous responses do not block the current codes. Rather, it keeps proceeding along with another task in the meantime. 

The best approach for handling asynchronous response is to access the similar API resource single or multiple times once the object creation gets completed.

Conclusion

JavaScript is one of the single-threaded programming languages. It implies that a single thing can perform at a time. But using the asynchronous method, one can run long waiting for network requests without interrupting the main thread. 

Above, we have detailed how to return the response from an asynchronous call. This will definitely be going to help JavaScript users to perform multiple network requests effectively.

Hope you like the blog. If you still have a query(s), comment in the below section, I will guide you in the best possible way. To get more conceptual details about JavaScript, keep reading GUVI Blogs.

Answer below MCQs and test what you have learned from this blog!!!

1. What would be the result if the user called setTimeout() with the 0ms time?

(A) Execute immediately

(B) Keep running continuously

(C) Place in queue

(D) Place in the stack

Correct Answer: (C)
With the time of 0ms for setTimeout, the function will not get invoked immediately. Rather, it will be placed in a queue to get invoked “as soon as possible” once the main event handler finishes running.

2. To which object, does the setTimeout() function belongs?

(A) Event

(B) Location

(C) Element

(D) Window

Correct Answer: (D)
The window’s setTimeout() function schedules the function to get run after the particular milliseconds’ pass.

3. Check whether the statement is TRUE or FALSE.

The callbacks() are useful for executing a function just after the other functions end.

(A) True

(B) False

Correct Answer: (A)
So, with the help of callbacks(), the user can execute the function just after the other function ends.

4. Check if the statement is True or False.

We can use await only and only with promises.

(A) True

(B) False

Correct Answer: (B)
This statement is not true. JS users can use await with promises and objects, which implements the .then function.

5. Which of the following is the state(s) of a promise?

  1. Rejected
  2. Fulfilled
  3. Pending
  4. All of above
Correct Answer: (D)
The promise has the rejected, fulfilled, and pending states.

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