Async Await Javascript Interview Questions

Async/await is a way of handling asynchronous tasks in a synchronous manner. This means that the CPU will not be blocked while waiting for an asynchronous task to complete. This can help improve performance by allowing the CPU to continue working on other tasks while the asynchronous task is being handled.

Async/await can help improve performance and scalability by allowing your application to do other work while it is waiting for a task to complete. This can help avoid bottlenecks and keep your application responsive. Additionally, using async/await can help reduce the overall amount of code needed to be written, making your application easier to maintain.

Asynchronous programming is a form of programming that allows for tasks to be completed out of order. This means that a program can start a task and then move on to other tasks before the first one is completed. This can be helpful in situations where a task might take a long time to complete, but the program doesn’t need to wait for it to finish before moving on.

Yes, it is possible to use async/await with promise chains. In order to do this, you must first await the initial promise in the chain, and then you can continue working with the results of that promise as if they were synchronous. This can be a useful way to avoid callback hell when working with asynchronous code.

This is due to the way that async/await functions work. When an async function is called, it will return immediately, even if the actual work is not yet finished. This is why callback functions are often used with async functions, so that the callback can be called when the work is actually finished.


When we pass multiple promises to the Promise.race method, it resolves/rejects the first promise that resolves/rejects. To the setTimeout method, we pass a timer: 500ms for the first promise (firstPromise), and 100ms for the second promise (secondPromise). This means that the secondPromise resolves first with the value of ‘two’. res now holds the value of ‘two’, which gets logged. The correct answer is B.

  • A: "I made it!"
  • B: Promise {: "I made it!"}
  • C: Promise {}
  • D: undefined
  • The main difference is that Task is a class while ValueTask is a struct. Latter was added to the BCL to alleviate unnecessary pressure on the garbage collector caused by asynchronous methods that often return synchronously (e.g. cached result). You can think of ValueTask as a discriminated union of Task or a synchronous result. Since recently, ValueTask can also represent a result signaled by ValueTaskSource.

    A good example of an asynchronous operation is reading a file from a hard drive. To read individual bytes of a file, operating system issues requests to the driver software, which in turn tells the drive to seek to a specific position by moving its mechanical head. The process of moving the head around is asynchronous, its not an operation that runs on CPU, its just a physical task you have to wait for completing. This is the type of “pure” async operation that can be represented with the async/await pattern.

    That said, an asynchronous task may also represent some CPU-bound calculation happening on a separate thread, but this is an implementation detail. This is useful when you want to delegate execution to a different thread in order to not block the calling thread, while disguising it as an asynchronous operation. You can do that by calling Task.Run().

    For compatibility reasons, due to constraints when working with Windows UI and classic ASP.NET applications. In one you can only interact with controls from the main thread, in the other you can only serve the response on the same thread that handled the request, so it was important that the execution continued on the captured context.

    Upon reaching the await keyword, the runtime will return control back to the calling method, which may or may not await it. If it does await, the same happens again, returning control to the caller of that method in turn, until it reaches a method in the call stack that either does not await (usually event handlers on a message loop thread) or materializes the task synchronously (e.g. entry point in a console application).

    JS Interview – Async – Question 2

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