JavaScript Promises and Async Quiz

1. What are the three states a Promise can be in?

Pending, resolved, rejected

Pending, fulfilled, rejected

Waiting, completed, failed

Initial, success, error

A Promise in JavaScript can be in one of three states: "pending," which indicates that the operation is still ongoing; "fulfilled," meaning the operation completed successfully; and "rejected," which signifies that the operation failed. These states help manage asynchronous operations effectively, allowing developers to handle outcomes appropriately.

Explanation

A Promise in JavaScript can be in one of three states: "pending," which indicates that the operation is still ongoing; "fulfilled," meaning the operation completed successfully; and "rejected," which signifies that the operation failed. These states help manage asynchronous operations effectively, allowing developers to handle outcomes appropriately.

2. Which method is used to handle both fulfilled and rejected promises?

Then()

Catch()

Finally()

Resolve()

The `finally()` method is used to execute code after a promise is settled, regardless of whether it was fulfilled or rejected. This ensures that any necessary cleanup or final actions occur without being dependent on the outcome of the promise, providing a way to handle both scenarios in a unified manner.

Explanation

The `finally()` method is used to execute code after a promise is settled, regardless of whether it was fulfilled or rejected. This ensures that any necessary cleanup or final actions occur without being dependent on the outcome of the promise, providing a way to handle both scenarios in a unified manner.

3. What does the async keyword do when placed before a function?

Makes the function run in parallel

Automatically returns a Promise

Speeds up function execution

Prevents error handling

The async keyword transforms a function so that it always returns a Promise, regardless of the return value. If the function returns a value, it is wrapped in a resolved Promise, and if it throws an error, it results in a rejected Promise. This allows for easier handling of asynchronous operations using await.

Explanation

The async keyword transforms a function so that it always returns a Promise, regardless of the return value. If the function returns a value, it is wrapped in a resolved Promise, and if it throws an error, it results in a rejected Promise. This allows for easier handling of asynchronous operations using await.

4. The await keyword can only be used inside an ______ function.

The await keyword is used to pause the execution of an asynchronous function until a Promise is resolved. It can only be utilized within an async function, which allows for cleaner, more readable asynchronous code by enabling the use of standard control flow constructs like try/catch and if/else without deeply nested callbacks.

Explanation

The await keyword is used to pause the execution of an asynchronous function until a Promise is resolved. It can only be utilized within an async function, which allows for cleaner, more readable asynchronous code by enabling the use of standard control flow constructs like try/catch and if/else without deeply nested callbacks.

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5. What is the correct syntax to catch an error in an async function?

Async function test() { await something().catch() }

Async function test() { try { await something() } catch(e) {} }

Async function test() { if (await something()) error() }

Async function test() { await something() || error() }

Using a try-catch block in an async function allows you to handle errors that may occur during the execution of awaited promises. The try block contains the code that might throw an error, while the catch block captures and processes that error, ensuring the program can continue running smoothly without crashing.

Explanation

Using a try-catch block in an async function allows you to handle errors that may occur during the execution of awaited promises. The try block contains the code that might throw an error, while the catch block captures and processes that error, ensuring the program can continue running smoothly without crashing.

6. How do you create a Promise that is immediately resolved with a value?

Promise.create(value)

Promise.resolve(value)

New Promise(value)

Promise.then(value)

To create a Promise that is immediately resolved with a specified value, you use `Promise.resolve(value)`. This method returns a Promise object that is resolved with the given value, allowing for easier handling of asynchronous operations without needing to manually create a new Promise and resolve it.

Explanation

To create a Promise that is immediately resolved with a specified value, you use `Promise.resolve(value)`. This method returns a Promise object that is resolved with the given value, allowing for easier handling of asynchronous operations without needing to manually create a new Promise and resolve it.

7. What does Promise.all() return if one promise rejects?

An array of results

A rejected promise

Null

Undefined

Promise.all() is designed to handle multiple promises simultaneously. If any of the promises within the array reject, the entire operation fails, and Promise.all() returns a rejected promise. This behavior ensures that you can easily manage errors when dealing with multiple asynchronous operations, as it allows you to catch the rejection in a single error handler.

Explanation

Promise.all() is designed to handle multiple promises simultaneously. If any of the promises within the array reject, the entire operation fails, and Promise.all() returns a rejected promise. This behavior ensures that you can easily manage errors when dealing with multiple asynchronous operations, as it allows you to catch the rejection in a single error handler.

8. Which method returns a promise that resolves with the first fulfilled promise?

Promise.all()

Promise.race()

Promise.any()

Promise.first()

Promise.any() returns a promise that resolves as soon as any of the promises in the iterable fulfill. If at least one promise is fulfilled, it resolves with the value of the first fulfilled promise. If no promises fulfill, it rejects with an AggregateError, indicating that all promises were rejected.

Explanation

Promise.any() returns a promise that resolves as soon as any of the promises in the iterable fulfill. If at least one promise is fulfilled, it resolves with the value of the first fulfilled promise. If no promises fulfill, it rejects with an AggregateError, indicating that all promises were rejected.

9. In a Promise chain, what does the then() method return?

In a Promise chain, the `then()` method returns a new Promise. This allows for chaining multiple `then()` calls, enabling the execution of asynchronous operations in sequence. Each `then()` can handle the resolved value of the previous Promise, allowing for streamlined error handling and a more manageable flow of asynchronous code.

Explanation

In a Promise chain, the `then()` method returns a new Promise. This allows for chaining multiple `then()` calls, enabling the execution of asynchronous operations in sequence. Each `then()` can handle the resolved value of the previous Promise, allowing for streamlined error handling and a more manageable flow of asynchronous code.

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10. True or False: An async function can be paused and resumed at await expressions.

True

False

An async function can indeed be paused at await expressions. When the execution reaches an await, the function yields control back to the event loop, allowing other operations to run. Once the awaited promise resolves, the function resumes execution from that point, continuing with the subsequent code. This behavior enables efficient handling of asynchronous operations.

Explanation

An async function can indeed be paused at await expressions. When the execution reaches an await, the function yields control back to the event loop, allowing other operations to run. Once the awaited promise resolves, the function resumes execution from that point, continuing with the subsequent code. This behavior enables efficient handling of asynchronous operations.

11. What happens if you forget to await a Promise in an async function?

The function waits automatically

The function continues without waiting

An error is thrown

The promise is cancelled

When you forget to await a Promise in an async function, the function will not pause for the Promise to resolve. Instead, it will continue executing the subsequent code immediately, potentially leading to unexpected behavior if the Promise's result is needed later in the function.

Explanation

When you forget to await a Promise in an async function, the function will not pause for the Promise to resolve. Instead, it will continue executing the subsequent code immediately, potentially leading to unexpected behavior if the Promise's result is needed later in the function.

12. How do you handle multiple async operations in parallel?

Use nested await statements

Use Promise.all() with an array of promises

Use setTimeout()

Use multiple try-catch blocks

Using Promise.all() allows you to execute multiple asynchronous operations concurrently, waiting for all of them to complete before proceeding. This method takes an array of promises and resolves when all promises are fulfilled or rejects if any promise fails, making it efficient for handling parallel tasks in JavaScript.

Explanation

Using Promise.all() allows you to execute multiple asynchronous operations concurrently, waiting for all of them to complete before proceeding. This method takes an array of promises and resolves when all promises are fulfilled or rejects if any promise fails, making it efficient for handling parallel tasks in JavaScript.