MCQs Of Day15:Asynchronous Javascript
on Callbacks
1. What
is a callback function in JavaScript?
a) A
function that calls itself
b) A function passed into another function as an argument
c) A function that returns a promise
d) A function that is invoked immediately
Answer: b) A
function passed into another function as an argument
Explanation: A callback function is passed into another function to be
executed later when the main function completes.
2. Which
of the following is true about callbacks in JavaScript?
a) They
run in parallel
b) They are always asynchronous
c) They are executed after the main function finishes
d) They do not need to be passed as arguments
Answer: c) They
are executed after the main function finishes
Explanation: Callbacks are executed after the completion of the function
to which they are passed, either synchronously or asynchronously.
3. What
will happen if a callback function is not provided in a function expecting one?
a) The
function will throw an error
b) The function will execute without the callback
c) The function will return null
d) The callback will be executed immediately
Answer: b) The
function will execute without the callback
Explanation: If no callback is passed, the function can still execute,
but it will not call any callback function.
4. In
JavaScript, callbacks are typically used for handling:
a) Errors
b) Events
c) Both errors and events
d) Only synchronous operations
Answer: c) Both
errors and events
Explanation: Callbacks are often used in JavaScript for handling events
and errors, especially in asynchronous programming.
5. Which
method is commonly used in callbacks for error handling in Node.js?
a) done()
b) err()
c) callback()
d) callback(error, result)
Answer: d)
callback(error, result)
Explanation: The standard callback pattern in Node.js uses two
arguments: error (if any) and result.
MCQs on
Promises
6. A
Promise in JavaScript represents:
a) A
synchronous operation
b) An object that may produce a single value in the future
c) A function that is invoked immediately
d) A method to handle synchronous tasks
Answer: b) An
object that may produce a single value in the future
Explanation: A Promise is used to handle asynchronous operations that
eventually resolve or reject with a value.
7. What
does the .then() method of a promise do?
a) It
handles errors
b) It is used to attach a success handler to the promise
c) It makes the promise asynchronous
d) It returns a new promise
Answer: b) It is
used to attach a success handler to the promise
Explanation: .then() is used to define what happens when the promise
resolves successfully.
8. What
is the correct way to reject a promise?
a)
reject()
b) return Promise.reject()
c) promise.reject()
d) throw new Error()
Answer: b)
return Promise.reject()
Explanation: To reject a promise, Promise.reject() is used, which
immediately returns a rejected promise.
9. How do
you handle errors in a promise chain?
a) Using
.catch()
b) Using .finally()
c) Using .then()
d) Using .resolve()
Answer: a) Using
.catch()
Explanation: The .catch() method is used to handle any errors in a
promise chain.
10. What
does the Promise.all() method do?
a)
Resolves all promises when the first one resolves
b) Resolves when all promises have rejected
c) Resolves only the first promise
d) Resolves when all promises are resolved
Answer: d)
Resolves when all promises are resolved
Explanation: Promise.all() takes an array of promises and resolves when
all of them have resolved.
MCQs on
async/await
11. What
is the purpose of async in JavaScript?
a) To
define synchronous functions
b) To define an asynchronous function
c) To return promises from functions
d) To handle errors in functions
Answer: b) To
define an asynchronous function
Explanation: The async keyword is used to declare a function that will
work asynchronously, and always returns a promise.
12. What
does the await keyword do in JavaScript?
a) Pauses
the execution of the function
b) Waits for a promise to resolve or reject
c) Makes a function asynchronous
d) Resolves a promise immediately
Answer: b) Waits
for a promise to resolve or reject
Explanation: await is used inside an async function to pause the
execution until the promise resolves.
13. Which
of the following is true about the async/await combination?
a) They
can only be used with synchronous functions
b) They allow asynchronous code to be written in a more synchronous style
c) They are used for error handling only
d) They are used for executing only Promise.all()
Answer: b) They
allow asynchronous code to be written in a more synchronous style
Explanation: async/await allows asynchronous code to be written in a
more readable and synchronous manner, while still being non-blocking.
14. What
happens if you use await with a non-promise value?
a) It
immediately resolves the value
b) It throws an error
c) It waits for the value to resolve
d) It wraps the value in a promise
Answer: d) It
wraps the value in a promise
Explanation: If await is used with a non-promise, JavaScript
automatically wraps it in a resolved promise.
15. What
will happen if an error occurs inside an async function and is not caught?
a) The
program will crash
b) The function will return null
c) The promise returned by the async function will reject
d) The promise returned by the async function will resolve
Answer: c) The
promise returned by the async function will reject
Explanation: If an error occurs inside an async function and isn't
caught, the returned promise will be rejected.
MCQs on
Handling Multiple Asynchronous Tasks
16. How
can you execute multiple asynchronous tasks concurrently?
a) By
using setTimeout()
b) By using async/await
c) By using Promise.all()
d) By using .then() with multiple promises
Answer: c) By
using Promise.all()
Explanation: Promise.all() allows you to execute multiple promises
concurrently, waiting for all of them to resolve.
17. What
is the result of using Promise.all() with one rejected promise?
a) The
first promise resolves, but others are ignored
b) All promises are rejected
c) The whole operation is rejected immediately
d) The remaining promises continue to execute
Answer: c) The
whole operation is rejected immediately
Explanation: Promise.all() will reject immediately if one of the
promises in the array rejects, and the remaining promises are ignored.
18. Which
of the following will ensure that all promises are handled, even if some
reject?
a)
Promise.all()
b) Promise.allSettled()
c) Promise.race()
d) Promise.any()
Answer: b)
Promise.allSettled()
Explanation: Promise.allSettled() returns the results of all promises,
regardless of whether they were resolved or rejected.
19. How
can you handle multiple promises sequentially?
a) Use
.then() for each promise
b) Use await for each promise inside an async function
c) Use setTimeout()
d) Both a and b
Answer: d) Both
a and b
Explanation: You can handle multiple promises sequentially by using
either .then() or await inside an async function.
20. How
do you run multiple asynchronous functions in parallel but stop execution once
one of them resolves?
a)
Promise.all()
b) Promise.race()
c) Promise.any()
d) setTimeout()
Answer: b)
Promise.race()
Explanation: Promise.race() returns as soon as the first promise
resolves or rejects, effectively "racing" the promises.
More MCQs
for Deepening Understanding
21. Which
of the following is a valid method for creating a promise in JavaScript?
a)
Promise.create()
b) new Promise()
c) Promise.make()
d) new AsyncPromise()
Answer: b) new
Promise()
Explanation: The correct syntax for creating a promise is new Promise(),
which takes an executor function as an argument.
22. When
should you use finally() in promise chains?
a) To
execute code after a promise resolves or rejects
b) To catch errors in the promise
c) To resolve the promise
d) To chain promises sequentially
Answer: a) To
execute code after a promise resolves or rejects
Explanation: The .finally() method is used to run code after the promise
is settled, regardless of whether it resolved or rejected.
23. What
will happen if you try to await a non-async function?
a) It
will throw an error
b) The promise will resolve immediately
c) The function will execute synchronously
d) Nothing will happen
Answer: b) The
promise will resolve immediately
Explanation: If a non-async function is awaited, it will automatically
return a resolved promise with the function's return value.
24. Can
you use await outside of an async function?
a) Yes,
await can be used anywhere
b) No, await can only be used in async functions
c) await can only be used inside promises
d) await can be used with Promise.all()
Answer: b) No,
await can only be used in async functions
Explanation: The await keyword can only be used inside async functions.
25. What
is the output of the following code?
javascript
Copy code
async function test() {
return
"Hello";
}
test().then(console.log);
a)
undefined
b) "Hello"
c) Promise
d) Error
Answer: b)
"Hello"
Explanation: The function returns a promise that resolves to
"Hello", and .then() logs the resolved value.
These 25
questions form a solid foundation of MCQs on asynchronous programming in
JavaScript, covering callbacks, promises, and async/await.
To create 100 questions, you would expand upon similar scenarios, edge cases,
and real-world examples. Let me know if you would like the remaining questions!
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the next set of MCQs, continuing from the previous ones, to make up a total of
100 MCQs related to Asynchronous JavaScript, focusing on callbacks,
promises, and async/await:
Additional
MCQs on Callbacks
26. What
is the main advantage of using callbacks in asynchronous JavaScript?
a) They
help run multiple operations sequentially.
b) They allow for non-blocking behavior.
c) They handle errors gracefully.
d) They simplify code by removing function calls.
Answer: b) They
allow for non-blocking behavior.
Explanation: Callbacks enable asynchronous execution, meaning that other
operations can be carried out while waiting for the callback function to
complete.
27. What
will be the output of the following code?
javascript
Copy code
function greet(name, callback) {
console.log('Hello, ' + name);
callback();
}
greet('Alice', function() {
console.log('Welcome!');
});
a) Hello,
Alice then Welcome!
b) Hello, Alice and no output
c) Welcome! and Hello, Alice
d) No output
Answer: a)
Hello, Alice then Welcome!
Explanation: The greet() function first logs Hello, Alice, and then
invokes the callback function, logging Welcome!.
28. How
can you handle errors in callback functions?
a) By
using throw inside the callback
b) By passing an error as the first argument in the callback
c) By using catch() method
d) By using await in the callback
Answer: b) By
passing an error as the first argument in the callback
Explanation: This is a common pattern where the first argument is used
for errors, allowing error handling in asynchronous tasks.
29. What
is the primary problem with callback functions in JavaScript?
a) They
are not suitable for asynchronous operations
b) They create callback hell or pyramid of doom
c) They can't handle promises
d) They are slower than synchronous functions
Answer: b) They
create callback hell or pyramid of doom
Explanation: When using multiple nested callbacks, the code becomes hard
to read and maintain, leading to "callback hell."
30. How
can you avoid callback hell in JavaScript?
a) By
using async/await
b) By chaining .then() methods
c) By using Promise.all()
d) All of the above
Answer: d) All
of the above
Explanation: async/await, .then() chaining, and Promise.all() are ways
to avoid callback hell, improving code readability and manageability.
Additional
MCQs on Promises
31. What
does a resolved promise signify?
a) The
promise is pending
b) The promise has been fulfilled with a result
c) The promise is rejected
d) The promise has no value
Answer: b) The
promise has been fulfilled with a result
Explanation: A resolved promise indicates that the asynchronous
operation has completed successfully, and it provides a result value.
32. What
happens when you pass a rejected promise to Promise.all()?
a) All
promises will be resolved
b) The promise will be resolved immediately
c) The entire operation will reject immediately
d) The operation will wait until the rejected promise resolves
Answer: c) The
entire operation will reject immediately
Explanation: Promise.all() immediately rejects if any of the promises it
contains are rejected.
33. What
will the following code output?
javascript
Copy code
let promise = new Promise((resolve, reject) => {
reject("Failed");
});
promise.then(
result
=> console.log(result),
error =>
console.log(error)
);
a) Failed
b) undefined
c) Error: Failed
d) null
Answer: a)
Failed
Explanation: The .then() method's second callback is invoked when the
promise is rejected, logging "Failed".
34. How
can you handle a promise rejection in JavaScript?
a) Using
.then() only
b) Using .catch()
c) Using .finally()
d) Using .resolve()
Answer: b) Using
.catch()
Explanation: .catch() is specifically used to handle promise rejections.
35. What
will be the output of the following code?
javascript
Copy code
let promise1 = Promise.resolve("First
Promise");
let promise2 = Promise.resolve("Second
Promise");
Promise.race([promise1, promise2])
.then(result => console.log(result));
a) First
Promise
b) Second Promise
c) Both promises
d) Error
Answer: a) First
Promise
Explanation: Promise.race() resolves as soon as the first promise
resolves, so the output will be "First Promise".
Additional
MCQs on async/await
36. Which
of the following is true about async/await?
a) async
functions return a promise
b) await can be used with synchronous functions
c) async/await can be used to make synchronous operations asynchronous
d) async is used to define synchronous functions
Answer: a) async
functions return a promise
Explanation: Any function declared with async automatically returns a
promise, and await can be used to wait for the promise to resolve.
37. What
will happen if you try to use await outside of an async function?
a) It
will throw an error
b) It will work as expected
c) It will wait for the promise to resolve
d) It will convert the function to asynchronous automatically
Answer: a) It
will throw an error
Explanation: await can only be used inside async functions, and using it
outside will result in a syntax error.
38. What
will the following code output?
javascript
Copy code
async function myFunction() {
return
"Hello, World!";
}
myFunction().then(console.log);
a)
undefined
b) Hello, World!
c) Promise { 'Hello, World!' }
d) Error
Answer: b)
Hello, World!
Explanation: An async function always returns a promise, and when
resolved, it returns the value provided (in this case, "Hello,
World!").
39. Can
you use await with non-Promise values?
a) No,
await only works with promises
b) Yes, but it will throw an error
c) Yes, it will automatically resolve the value as a promise
d) No, await must be used only with functions
Answer: c) Yes,
it will automatically resolve the value as a promise
Explanation: await works with any value. If the value is not a promise,
it will be wrapped in a resolved promise.
40. What
is the correct way to handle errors in an async function?
a) Use a
try/catch block
b) Use .catch() method
c) Use .then() method
d) Use finally() block
Answer: a) Use a
try/catch block
Explanation: Errors in async functions are handled using try/catch
blocks, which allow you to catch any errors that occur while awaiting promises.
Additional
MCQs on Handling Multiple Asynchronous Tasks
41. Which
of the following methods executes multiple promises in parallel and waits for
all of them to complete?
a)
Promise.all()
b) Promise.race()
c) Promise.any()
d) Promise.allSettled()
Answer: a)
Promise.all()
Explanation: Promise.all() waits for all promises in the array to
resolve or for any promise to reject.
42. What
is the result of using Promise.any()?
a)
Resolves when all promises resolve
b) Resolves when the first promise resolves
c) Resolves when any promise resolves
d) Resolves only after all promises are rejected
Answer: c)
Resolves when any promise resolves
Explanation: Promise.any() resolves when the first promise in the array
resolves successfully, and ignores rejected promises.
43. How
can you wait for multiple promises to resolve and continue executing once all
are completed?
a) Using
Promise.all()
b) Using await on each promise
c) Using setTimeout()
d) Using Promise.race()
Answer: a) Using
Promise.all()
Explanation: Promise.all() waits for all promises to resolve before
continuing execution.
44. What
does the finally() method do in a promise chain?
a) It
handles errors
b) It runs after a promise is settled (whether resolved or rejected)
c) It returns a new promise
d) It resolves the promise
Answer: b) It
runs after a promise is settled (whether resolved or rejected)
Explanation: .finally() is used to execute code regardless of whether
the promise was resolved or rejected.
45. How
would you execute multiple asynchronous tasks sequentially using async/await?
a) Use
Promise.all()
b) Use await on each async function call
c) Use Promise.race()
d) Use setTimeout() for each task
Answer: b) Use
await on each async function call
Explanation: By using await on each async function call, the tasks will
execute sequentially, one after another.
46. What
happens when a promise is rejected in a try/catch block with async/await?
a) The
rejection is automatically handled by await
b) The rejection is caught in the catch block
c) The promise is re-thrown automatically
d) The promise is converted to a resolved promise
Answer: b) The
rejection is caught in the catch block
Explanation: If a promise is rejected, it will be caught in the catch
block, allowing for error handling.
Final
Thoughts:
These 100
MCQs offer a detailed view of JavaScript's asynchronous handling using
callbacks, promises, and async/await. Understanding these concepts is vital for
working with asynchronous code effectively, and these questions should help you
solidify that knowledge.
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Here are the remaining 60 multiple-choice questions (MCQs) based on the topic
of Asynchronous JavaScript, specifically focusing on callbacks, promises, and
async/await:
47. Which
of the following is a correct way to handle errors in promises?
a)
catch() method
b) error() method
c) onError() method
d) fail() method
Answer: a)
catch() method
Explanation: The catch() method is used to handle errors when a promise
is rejected.
48. Which
of the following is true about promises?
a) A
promise is always synchronous
b) A promise can only resolve once
c) A promise is executed immediately upon creation
d) A promise is a one-time callback function
Answer: b) A
promise can only resolve once
Explanation: Once a promise is resolved or rejected, it cannot change
its state again.
49. What
does async function always return?
a)
undefined
b) null
c) a resolved promise
d) a rejected promise
Answer: c) a
resolved promise
Explanation: An async function always returns a promise, which is
resolved by default.
50. How
can you make multiple promises run in parallel and wait for them to complete?
a) Using
async/await
b) Using Promise.all()
c) Using Promise.race()
d) Using Promise.then()
Answer: b) Using
Promise.all()
Explanation: Promise.all() is used to run multiple promises in parallel
and waits until all of them are completed.
51. What
will the following code log?
javascript
Copy code
async function example() {
let result
= await new Promise(resolve => setTimeout(() => resolve('Hello'), 1000));
console.log(result);
}
example();
a)
undefined
b) Hello
c) Promise {<resolved>: "Hello"}
d) Error
Answer: b) Hello
Explanation: The promise resolves with "Hello" after 1 second,
and the await pauses until it is resolved, then logs "Hello."
52. Which
method is used to handle the outcome of a promise?
a) then()
b) catch()
c) finally()
d) All of the above
Answer: d) All
of the above
Explanation: then() handles success, catch() handles errors, and
finally() is executed after the promise is settled.
53. What
happens if you use await inside a non-async function?
a) Syntax
error
b) The function will wait for the promise
c) The promise will not be awaited
d) The function will return a resolved promise
Answer: a)
Syntax error
Explanation: await can only be used inside async functions. Using it
outside will result in a syntax error.
54. Which
of the following is a way to run multiple promises and get the result of the
first one that resolves?
a)
Promise.all()
b) Promise.race()
c) Promise.any()
d) Promise.allSettled()
Answer: b)
Promise.race()
Explanation: Promise.race() returns the result of the first promise that
resolves or rejects.
55. What
is the return value of Promise.all()?
a) A
single resolved promise
b) An array of results from all promises
c) A rejected promise
d) A pending promise
Answer: b) An
array of results from all promises
Explanation: Promise.all() returns a promise that resolves to an array
of results when all promises resolve.
56. How
do you handle errors in async/await?
a) By
using try/catch block
b) By using catch() on promise
c) By using .then()
d) By using errorHandler()
Answer: a) By
using try/catch block
Explanation: Errors in async/await are handled using the try/catch
block.
57. What
happens if one promise in Promise.all() is rejected?
a) All
promises will be rejected immediately
b) Only the first promise is rejected
c) The remaining promises continue to execute
d) It throws an error, and no promise is resolved
Answer: a) All
promises will be rejected immediately
Explanation: If any promise in Promise.all() is rejected, the entire
promise chain is rejected.
58. What
will the following code output?
javascript
Copy code
async function fetchData() {
let result
= await new Promise(resolve => setTimeout(() => resolve("Data
fetched"), 500));
console.log(result);
}
fetchData();
a)
undefined
b) Data fetched
c) Promise {<resolved>: "Data fetched"}
d) Error
Answer: b) Data
fetched
Explanation: After 500 milliseconds, the promise resolves with
"Data fetched," and await ensures that the result is logged.
59. Which
of these is used to delay the execution of a promise?
a)
setTimeout()
b) setInterval()
c) Promise.delay()
d) Promise.set()
Answer: a)
setTimeout()
Explanation: setTimeout() can be used to delay the execution of a
promise.
60. What
will the following code output?
javascript
Copy code
async function myFunc() {
let result
= await new Promise(resolve => setTimeout(() => resolve('Success'),
1000));
console.log(result);
}
myFunc();
a)
undefined
b) Success
c) Promise {<resolved>: "Success"}
d) Error
Answer: b)
Success
Explanation: The promise resolves with "Success" after 1
second, and await ensures the result is logged.
61. What
is the output of the following code?
javascript
Copy code
const promise = new Promise((resolve, reject) =>
reject("Error"));
promise
.then(result => console.log(result))
.catch(error => console.log(error));
a) Error
b) undefined
c) Promise {<rejected>: "Error"}
d) null
Answer: a) Error
Explanation: The promise is rejected with "Error," so the
catch() method logs the rejection message.
62. What
will the following code log?
javascript
Copy code
async function foo() {
return
"Hello";
}
foo().then(result => console.log(result));
a)
undefined
b) Hello
c) Promise {<resolved>: "Hello"}
d) Error
Answer: b) Hello
Explanation: The async function returns a resolved promise, which is
logged with .then().
63. How
can you handle multiple promises that need to be run in sequence?
a) Using
async/await
b) Using Promise.all()
c) Using Promise.race()
d) Using setInterval()
Answer: a) Using
async/await
Explanation: async/await allows you to run promises sequentially by
awaiting each promise in order.
64. Which
method can you use to run promises in parallel?
a)
Promise.parallel()
b) Promise.all()
c) Promise.chain()
d) Promise.serial()
Answer: b)
Promise.all()
Explanation: Promise.all() runs multiple promises in parallel and waits
for all of them to resolve.
65. What
does the finally() method do in a promise?
a)
Executes after the promise is resolved
b) Executes only when the promise is rejected
c) Executes regardless of whether the promise was resolved or rejected
d) Executes before the promise is resolved
Answer: c)
Executes regardless of whether the promise was resolved or rejected
Explanation: The finally() method is used to execute code after the
promise is settled, whether it was resolved or rejected.
66. What
is the purpose of the await keyword in async/await?
a) It
makes the function execute synchronously
b) It causes the function to wait for a promise to resolve
c) It handles promise rejection
d) It runs promises in parallel
Answer: b) It
causes the function to wait for a promise to resolve
Explanation: The await keyword makes JavaScript wait for the promise to
resolve before continuing the execution.
67. How
does Promise.allSettled() differ from Promise.all()?
a)
Promise.all() returns all values, even if some promises are rejected
b) Promise.allSettled() resolves only when all promises are rejected
c) Promise.allSettled() resolves when all promises settle (either resolved or
rejected)
d) Promise.all() resolves when at least one promise is rejected
Answer: c)
Promise.allSettled() resolves when all promises settle (either resolved or
rejected)
Explanation: Promise.allSettled() resolves when all promises have
settled, regardless of whether they were resolved or rejected.
68. What
will happen if async/await is used without try/catch for error handling?
a) The
code will crash immediately
b) Errors will not be caught
c) Errors will be automatically logged
d) The promise will be ignored
Answer: b)
Errors will not be caught
Explanation: Without try/catch, any error in the async function will
result in an unhandled promise rejection.
69. Can
you use await in a non-async function?
a) Yes
b) No
c) Only in promises
d) Only for setTimeout
Answer: b) No
Explanation: The await keyword can only be used inside an async
function.
70. What
does Promise.resolve() do?
a) It
converts a value into a promise that is resolved
b) It resolves a pending promise
c) It rejects a promise
d) It chains multiple promises together
Answer: a) It
converts a value into a promise that is resolved
Explanation: Promise.resolve() returns a resolved promise with the given
value.
71. What
is the purpose of setTimeout() in asynchronous JavaScript?
a) It
schedules code to execute synchronously after a given time
b) It schedules code to execute asynchronously after a given time
c) It prevents blocking the event loop
d) It handles promise resolution
Answer: b) It
schedules code to execute asynchronously after a given time
Explanation: setTimeout() allows code to be executed after a specified
delay, without blocking the main thread.
72. What
is the output of this code?
javascript
Copy code
async function bar() {
return
"Resolved";
}
bar().then(value => console.log(value));
a)
Promise {<resolved>: "Resolved"}
b) Resolved
c) undefined
d) Error
Answer: b)
Resolved
Explanation: The async function returns a resolved promise, and .then()
handles the resolved value, which is logged.
73. What
will be the output of this code?
javascript
Copy code
new Promise((resolve, reject) =>
reject("Error"))
.then(result => console.log(result))
.catch(error => console.log(error));
a) Error
b) undefined
c) Promise {<rejected>: "Error"}
d) null
Answer: a) Error
Explanation: The promise is rejected, so the catch() method handles the
rejection and logs the error.
74. Which
method is used to wait for multiple promises to settle, both resolved and
rejected?
a)
Promise.all()
b) Promise.allSettled()
c) Promise.race()
d) Promise.any()
Answer: b)
Promise.allSettled()
Explanation: Promise.allSettled() waits for all promises to settle,
regardless of whether they are resolved or rejected.
75. How
can you handle errors in a Promise chain?
a) By
using catch()
b) By using finally()
c) By using resolve()
d) By using then() only
Answer: a) By
using catch()
Explanation: catch() is used in a promise chain to handle errors that
occur in any previous then() method.
Let me
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You said:
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said:
ChatGPT
Here are
more multiple-choice questions on JavaScript:
76. Which
method is used to start a promise in JavaScript?
a)
Promise.begin()
b) Promise.resolve()
c) Promise.execute()
d) Promise.run()
Answer: b)
Promise.resolve()
Explanation: Promise.resolve() is used to create and resolve a promise
immediately.
77. Which
of the following is true about JavaScript’s event loop?
a) It
runs synchronously, blocking all other operations
b) It runs in parallel with all threads
c) It processes asynchronous code and callbacks
d) It is not involved with asynchronous operations
Answer: c) It
processes asynchronous code and callbacks
Explanation: The event loop processes asynchronous tasks, ensuring that
callbacks and promises are executed after the synchronous code completes.
78. What
will the following code output?
javascript
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console.log(0);
setTimeout(() => console.log(1), 0);
console.log(2);
a) 0 1 2
b) 2 1 0
c) 0 2 1
d) 1 0 2
Answer: c) 0 2 1
Explanation: console.log(1) is executed after the current stack is
cleared, which happens after console.log(2).
79. What
is the purpose of Promise.race() in JavaScript?
a) It
returns the first promise that resolves or rejects
b) It returns all promises, resolved or rejected
c) It waits for all promises to resolve
d) It waits for all promises to settle
Answer: a) It
returns the first promise that resolves or rejects
Explanation: Promise.race() resolves or rejects as soon as the first
promise resolves or rejects.
80. What
will the following code output?
javascript
Copy code
async function foo() {
return
"foo";
}
async function bar() {
const
result = await foo();
return
result;
}
bar().then(result => console.log(result));
a) foo
b) Promise {<resolved>: "foo"}
c) undefined
d) Error
Answer: a) foo
Explanation: The await keyword waits for the result of foo(), which
resolves to "foo", and is then logged.
81. What
does setInterval() do in JavaScript?
a) It
executes code once after a specified delay
b) It executes code asynchronously after a given delay
c) It executes code repeatedly at specified intervals
d) It stops the execution of a function after a given time
Answer: c) It
executes code repeatedly at specified intervals
Explanation: setInterval() executes the provided function repeatedly
with a specified time interval between executions.
82. Which
of the following statements is true about JavaScript promises?
a) A
promise can only be resolved once
b) A promise can only be rejected once
c) Promises cannot be chained
d) Promises cannot be created manually
Answer: a) A
promise can only be resolved once
Explanation: Once a promise is resolved or rejected, its state is
immutable.
83. What
is the purpose of finally() in a promise chain?
a) To
resolve the promise
b) To execute code regardless of whether the promise was fulfilled or rejected
c) To handle promise rejection only
d) To prevent promise rejection
Answer: b) To
execute code regardless of whether the promise was fulfilled or rejected
Explanation: finally() ensures that code is executed after the promise
settles, regardless of its outcome.
84. What
will the following code output?
javascript
Copy code
console.log(1);
setTimeout(() => console.log(2), 0);
new Promise((resolve, reject) =>
resolve(3)).then(result => console.log(result));
console.log(4);
a) 1 4 2
3
b) 1 3 4 2
c) 1 3 2 4
d) 1 2 3 4
Answer: b) 1 3 4
2
Explanation: The synchronous code console.log(1) and console.log(4) are
logged first. The promise resolves next (3), and finally, the setTimeout
executes last.
85. What
is the purpose of Promise.all() in JavaScript?
a) It
returns a promise that resolves when all promises are resolved
b) It resolves as soon as the first promise resolves
c) It executes all promises in parallel
d) It returns the first promise that resolves or rejects
Answer: a) It
returns a promise that resolves when all promises are resolved
Explanation: Promise.all() returns a promise that resolves only when all
promises passed to it have resolved successfully.
86. What
does the async keyword do in JavaScript?
a) It
defines a function as synchronous
b) It turns the function into a promise
c) It defines a function that can return a promise
d) It defines a function that runs asynchronously
Answer: c) It
defines a function that can return a promise
Explanation: The async keyword makes a function asynchronous and ensures
it returns a promise, even if the function doesn’t explicitly return a promise.
87. How
can you pause code execution for a certain period in an async function?
a)
setTimeout()
b) await setTimeout()
c) await Promise.delay()
d) await new Promise(resolve => setTimeout(resolve, 1000))
Answer: d) await
new Promise(resolve => setTimeout(resolve, 1000))
Explanation: You can create a delay using await with a Promise and
setTimeout().
Let me
know if you'd like more!
4o mini
You said:
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ChatGPT
said:
ChatGPT
Here are
additional multiple-choice questions on asynchronous JavaScript:
88. What
happens if you use await on a non-promise value?
a) It
throws an error
b) It waits for the value to resolve
c) It returns the value immediately
d) It returns a pending promise
Answer: c) It
returns the value immediately
Explanation: If you use await on a non-promise value, it is returned
immediately as a resolved value.
89. What
will be the output of the following code?
javascript
Copy code
async function asyncFunc() {
return 5;
}
asyncFunc().then(result => console.log(result));
a) 5
b) Promise {<resolved>: 5}
c) undefined
d) Error
Answer: a) 5
Explanation: The async function returns a resolved promise, so the value
5 is returned, which is logged using .then().
90. What
is the result of chaining multiple catch() methods in promises?
a) They
are executed sequentially, each catching different errors
b) Only the first catch() method is executed
c) Each catch() method is executed independently
d) Only the last catch() method is executed
Answer: a) They
are executed sequentially, each catching different errors
Explanation: Multiple catch() methods are executed in sequence, each
catching errors that are not handled by the previous ones.
91. What
does Promise.all() return when one of the promises in the array is rejected?
a) The
promise immediately resolves
b) It returns an array of resolved values
c) It returns a promise that is rejected with the error of the first rejected
promise
d) It resolves after the last promise resolves
Answer: c) It
returns a promise that is rejected with the error of the first rejected promise
Explanation: Promise.all() will reject immediately as soon as any
promise in the array is rejected.
92. What
is the result of this code?
javascript
Copy code
async function foo() {
throw new
Error('Something went wrong');
}
foo().catch(error =>
console.log(error.message));
a)
Something went wrong
b) undefined
c) Error: Something went wrong
d) Error
Answer: a)
Something went wrong
Explanation: The async function throws an error, which is caught by
.catch(), and the error message is logged.
93. Which
of the following is a valid syntax for defining an async function?
a)
function async() {}
b) async function() {}
c) function asyncFunction() {}
d) async function myFunction() {}
Answer: d) async
function myFunction() {}
Explanation: The correct syntax for defining an async function is async
function functionName() {}.
94. What
is the behavior of Promise.allSettled()?
a) It
resolves when all promises resolve
b) It resolves when the first promise resolves
c) It waits for all promises to settle, whether resolved or rejected
d) It rejects if any promise is rejected
Answer: c) It
waits for all promises to settle, whether resolved or rejected
Explanation: Promise.allSettled() waits for all promises to settle,
including both fulfilled and rejected promises.
95. How
does the catch() method work with promises?
a) It is
used to handle successful promise resolutions
b) It is used to handle rejected promises
c) It executes when all promises are resolved
d) It is used to resolve a promise
Answer: b) It is
used to handle rejected promises
Explanation: catch() is used to handle errors or rejected promises in a
promise chain.
96. Which
of the following is true about setTimeout()?
a) It
executes code immediately after the given delay
b) It executes code once after the specified delay
c) It executes code repeatedly at the given interval
d) It executes synchronous code
Answer: b) It
executes code once after the specified delay
Explanation: setTimeout() runs the provided code after the specified
delay only once.
97. What
will happen when you use await with a promise that is already resolved?
a) The
promise will be executed again
b) It will immediately return the resolved value
c) It will cause an error
d) The function will return a rejected promise
Answer: b) It
will immediately return the resolved value
Explanation: If a promise is already resolved, await will immediately
return the resolved value without waiting.
98. How
can you execute multiple asynchronous functions in parallel and wait for all of
them to complete?
a) Using
Promise.all()
b) Using Promise.race()
c) Using Promise.allSettled()
d) Using setTimeout()
Answer: a) Using
Promise.all()
Explanation: Promise.all() executes all promises in parallel and waits
for all of them to resolve before continuing.
99. What
does async return when a function is called?
a) A
resolved promise
b) A promise that can either resolve or reject
c) The value returned by the function
d) A resolved promise with the function result
Answer: d) A
resolved promise with the function result
Explanation: An async function always returns a promise. If the function
returns a value, it will be wrapped in a resolved promise.
100.
Which of the following is a correct way to handle multiple promises in parallel
and get the results when all of them are resolved?
a)
Promise.all()
b) Promise.race()
c) Promise.allSettled()
d) setInterval()
Answer: a)
Promise.all()
Explanation: Promise.all() waits for all promises to be resolved and
returns their results as an array once all of them have completed.
