What is the JavaScript event loop?

The mechanism that monitors the call stack and moves tasks from the callback queue to the stack

A background process that parallelizes JavaScript execution across multiple CPU cores

A built-in garbage collection routine that clears unused variables from the heap during idle time

A network synchronization tool used to coordinate data between the browser and the server

What is a callback function?

A function passed into another function as an argument to be executed at a later time

A specialized function that automatically retries an operation if it fails to resolve

A reserved keyword used to return the result of an asynchronous operation to the global scope

A native method used to trigger the immediate termination of a running asynchronous process

What is callback hell?

A situation where deeply nested callbacks make code difficult to read and manage errors

An execution error occurring when a recursive function exceeds the maximum call stack size

A memory leak caused by retaining references to functions that are no longer being used

A race condition where multiple callbacks attempt to update the same global state simultaneously

An object representing the eventual completion or failure of an asynchronous operation

A synchronous wrapper that forces an asynchronous function to return an immediate value

A global event emitter used to broadcast state changes across different parts of an application

A browser-only API designed to handle communication between different windows or tabs

What does Promise.resolve() do?

Returns a Promise object that is resolved with a specified value or result

Immediately triggers the .then() block of every pending Promise in the current queue

Converts a synchronous function into an asynchronous one without changing its return type

Clears all rejected Promises from the microtask queue to prevent application crashes

What does Promise.reject() do?

Returns a Promise object that is rejected with a given reason or error message

Terminates the execution of the current script to prevent further async operations

Prevents a Promise from ever settling to ensure that its callbacks are never executed

Automatically triggers a retry of the asynchronous operation using backoff logic

What is an async function?

A function that always returns a Promise and allows the use of the await keyword

A specialized function that runs on a separate thread to avoid blocking the UI

A function that executes immediately and pauses the event loop until it completes

A reserved constructor used to create new asynchronous classes in modern JavaScript

What is the purpose of the await keyword?

Pauses the execution of an async function until a Promise settles and returns its result

Speeds up the execution of multiple Promises by running them in parallel threads

Directly converts a Promise into a synchronous value available in the global scope

Forces the event loop to prioritize the current task over all other pending microtasks

How does Promise.all() behave?

Fulfills when all input Promises fulfill, but rejects immediately if any Promise rejects

Fulfills as soon as the first Promise resolves, regardless of the state of others

Executes a list of asynchronous functions one after another in a strict sequential order

Returns an array of results only after every Promise has either fulfilled or rejected

What is the behavior of Promise.race()?

Returns a Promise that settles as soon as any of the input Promises fulfill or reject

Waits for all Promises to complete and returns the one that took the least time

Rejects only if all of the provided Promises fail to resolve within a given timeout

Prioritizes the fastest network request while cancelling all other slower operations

What is the microtask queue?

A high-priority queue for Promises and MutationObservers processed before the next macrotask

A low-priority queue used for UI rendering and handling user input events like clicks

A temporary storage area for variables that are passed between different Web Workers

A debugging tool used to track the memory allocation of small objects in the heap

How does setTimeout interact with the event loop?

It schedules a callback to be executed as a macrotask after the current stack and microtasks

It pauses the event loop for a set duration to ensure no other code executes

It injects a function directly into the microtask queue for immediate execution

It bypasses the event loop to execute code on a separate background system thread

What is error handling with async/await?

The use of try/catch blocks to capture and handle errors from awaited Promises

The mandatory use of the .onerror() property on all asynchronous function declarations

A global monitoring system that automatically suppresses all asynchronous rejections

The practice of returning null instead of throwing an error when an operation fails

What is the use case for Promise.allSettled()?

When you need to wait for all operations to complete regardless of their individual success or failure

When you want to stop the execution of a batch of Promises as soon as any one of them fails

When you need to find the fastest successful response from a group of mirrored API endpoints

When you are working with legacy callback-based code that does not support modern error objects

What is the behavior of Promise.any()?

It returns the first Promise that fulfills, and only rejects if every input Promise rejects

It returns the first Promise that settles, whether it was a success or a rejection

It combines the results of all fulfilled Promises into a single aggregate success object

It cancels all pending Promises once any of the input Promises has successfully resolved

What is the role of the Promise constructor?

It creates a new Promise where an executor function manually controls the resolve and reject calls

It is a specialized utility used to merge two existing Promises into a single new instance

It acts as a decorator that automatically adds timeout logic to any synchronous function

It defines a template for asynchronous classes to ensure they implement a standard .then() method

What is Promise chaining?

A pattern where each .then() returns a new Promise, allowing for sequential async operations

A method of linking multiple Promises together so they all resolve at exactly the same time

A memory optimization that allows multiple Promises to share the same underlying result object

A security feature that prevents a Promise from being accessed by unauthorized scripts

What is the unhandledrejection event?

A global event fired when a Promise is rejected and no rejection handler is attached to it

A syntax error triggered during development when a Promise constructor is missing an executor

A network event that occurs when a server fails to respond to a fetch request within the timeout

A cleanup routine that runs automatically to close connections when an async function crashes

What is the AbortController API?

An interface that allows you to abort one or more Web requests or asynchronous operations

A specialized debugger used to stop the execution of the event loop for inspection

A resource management tool that automatically deletes old Promises from the memory heap

A middleware component used to reject incoming HTTP requests based on their payload size

What is the primary function of the Fetch API?

A modern, Promise-based interface for making network requests and handling responses

A server-side utility used to scrape HTML content from external websites for indexing

A synchronous data retrieval method that blocks code execution until a file is downloaded

An internal browser routine that pre-loads images to improve the speed of page rendering

What is an async iterator?

An object that implements the Symbol.asyncIterator method for use with for-await-of loops

A function that automatically iterates through an array and executes each item in parallel

A specialized loop that allows synchronous functions to be executed inside an async context

A data structure that stores asynchronous results in a first-in, first-out sequence

What is the difference between concurrent and sequential execution?

Concurrent runs independent tasks in parallel; sequential waits for each task to finish before starting the next

Concurrent is always safer for database writes; sequential is faster for read-only operations

Concurrent execution requires Web Workers; sequential execution is the only mode for the main thread

Concurrent refers to nested callbacks; sequential refers to the use of modern async/await syntax

What is top-level await?

The ability to use the await keyword at the highest level of a module without an async function

A performance optimization that lifts all await calls to the top of the call stack for faster execution

A specialized error handling mode that catches all rejections at the top level of the window object

A global setting that forces all functions in a script to act as if they were declared as async

What is async context preservation?

Ensuring that "this" and other closure variables remain accessible across asynchronous boundaries

A memory management technique that keeps local variables from being garbage collected

A security policy that prevents asynchronous functions from accessing the global window scope

The practice of saving the current state of a Promise to a local file in case of a crash

What is the retry pattern in asynchronous code?

A strategy of wrapping async calls in a loop with error handling and exponential backoff

A recursive function that re-executes itself immediately whenever any variable changes state

A database configuration that automatically restores deleted records from a transaction log

An optimization that caches successful results to avoid making the same request twice

What is a typical use case for Promise.allSettled()?

Handling a batch of independent operations where partial success is acceptable and results must be tracked

Ensuring that a critical sequence of financial transactions stops immediately if one step fails

Improving the performance of a single large file upload by splitting it into smaller chunks

Translating synchronous XML requests into a modern JSON format for use in older browsers

What is the purpose of debouncing an asynchronous operation?

To limit the rate at which a function executes by waiting for a period of inactivity before triggering

To encrypt asynchronous data payloads before they are transmitted over a public network

To ensure that two different asynchronous operations do not access the same memory address

To automatically increase the priority of a Promise so it resolves faster under heavy load

How is Promise.then() return value transformed?

If the handler returns a value, the new Promise resolves to that value; if it returns a Promise, it waits for it

The return value is ignored, and the new Promise always resolves with the original value from the first Promise

The return value is immediately converted into a string to ensure type safety in the next link of the chain

Returning any value from a .then() block causes the entire Promise chain to reject with a TypeError

What are asynchronous cleanup patterns?

Using try/finally blocks to ensure that resources like timers and connections are closed regardless of success

The use of a global garbage collector that identifies and deletes pending Promises every sixty seconds

A specialized syntax that automatically deletes all variables at the end of every async function

A process of overwriting sensitive data with null values before a Promise is allowed to settle

What are generator functions?

Functions that can be exited and later re-entered, with their context preserved across multiple re-entries

Specialized constructors used to generate random data sets for performance and load testing

An internal engine routine that generates optimized machine code from high-level JavaScript

A type of asynchronous function that automatically parallelizes loops for increased performance

Quizzes

Next in BackendDatabase Design & Patterns

Backend JavaScript

Async JavaScript: Promises, Async/Await, Event Loop

Mohammad Abu Mattar20 QuestionsIntermediatePublished: 12 Mar, 2026

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Async JavaScript: Promises, Async/Await, Event Loop

Up to 20 questions, shuffled on every run

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Welcome to the Async JavaScript quiz! Test your knowledge of Promises, async/await, the event loop, and advanced concurrency patterns. Each question has a hint and detailed explanations for all options. Good luck!

Answer key and explanations20 questions

The quiz above draws 20 questions at random from these 30, so a second attempt will not be the same run. Everything in the pool is listed here.

What is the JavaScript event loop?
AnswerThe mechanism that monitors the call stack and moves tasks from the callback queue to the stack
What is a callback function?
AnswerA function passed into another function as an argument to be executed at a later time
What is callback hell?
AnswerA situation where deeply nested callbacks make code difficult to read and manage errors
What is a Promise?
AnswerAn object representing the eventual completion or failure of an asynchronous operation
What does Promise.resolve() do?
AnswerReturns a Promise object that is resolved with a specified value or result
What does Promise.reject() do?
AnswerReturns a Promise object that is rejected with a given reason or error message
What is an async function?
AnswerA function that always returns a Promise and allows the use of the await keyword
What is the purpose of the await keyword?
AnswerPauses the execution of an async function until a Promise settles and returns its result
How does Promise.all() behave?
AnswerFulfills when all input Promises fulfill, but rejects immediately if any Promise rejects
What is the behavior of Promise.race()?
AnswerReturns a Promise that settles as soon as any of the input Promises fulfill or reject
What is the microtask queue?
AnswerA high-priority queue for Promises and MutationObservers processed before the next macrotask
How does setTimeout interact with the event loop?
AnswerIt schedules a callback to be executed as a macrotask after the current stack and microtasks
What is error handling with async/await?
AnswerThe use of try/catch blocks to capture and handle errors from awaited Promises
What is the use case for Promise.allSettled()?
AnswerWhen you need to wait for all operations to complete regardless of their individual success or failure
What is the behavior of Promise.any()?
AnswerIt returns the first Promise that fulfills, and only rejects if every input Promise rejects
What is the role of the Promise constructor?
AnswerIt creates a new Promise where an executor function manually controls the resolve and reject calls
What is Promise chaining?
AnswerA pattern where each .then() returns a new Promise, allowing for sequential async operations
What is the unhandledrejection event?
AnswerA global event fired when a Promise is rejected and no rejection handler is attached to it
What is the AbortController API?
AnswerAn interface that allows you to abort one or more Web requests or asynchronous operations
What is the primary function of the Fetch API?
AnswerA modern, Promise-based interface for making network requests and handling responses
What is an async iterator?
AnswerAn object that implements the Symbol.asyncIterator method for use with for-await-of loops
What is the difference between concurrent and sequential execution?
AnswerConcurrent runs independent tasks in parallel; sequential waits for each task to finish before starting the next
What is top-level await?
AnswerThe ability to use the await keyword at the highest level of a module without an async function
What is async context preservation?
AnswerEnsuring that "this" and other closure variables remain accessible across asynchronous boundaries
What is the retry pattern in asynchronous code?
AnswerA strategy of wrapping async calls in a loop with error handling and exponential backoff
What is a typical use case for Promise.allSettled()?
AnswerHandling a batch of independent operations where partial success is acceptable and results must be tracked
What is the purpose of debouncing an asynchronous operation?
AnswerTo limit the rate at which a function executes by waiting for a period of inactivity before triggering
How is Promise.then() return value transformed?
AnswerIf the handler returns a value, the new Promise resolves to that value; if it returns a Promise, it waits for it
What are asynchronous cleanup patterns?
AnswerUsing try/finally blocks to ensure that resources like timers and connections are closed regardless of success
What are generator functions?
AnswerFunctions that can be exited and later re-entered, with their context preserved across multiple re-entries