Promises in JavaScript Overview

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Questions and Answers

What is the purpose of the `resolve` function in a Promise?

To create a new Promise object

To register callbacks for handling the result

To emit the result of a successful operation (correct)

To emit the error from a failed operation

Which function is called when a Promise is rejected?

onReject (correct)

handleError

onResolve

onComplete

What does the `then` method return when called on a Promise?

An error object

The resolved value of the Promise

A boolean value indicating success

Another Promise (correct)

What will be logged to the console if the Promise resolves with 'Success!'?

Success! Signup and view all the answers

In the provided Promise example, what condition leads to calling `reject`?

Math.random() returns a value less than or equal to 0.5 Signup and view all the answers

What is a fundamental feature of the Promise interface in the provided code example?

It returns a Promise object that can resolve or reject. Signup and view all the answers

How does the 'first' function differ when using callbacks compared to using Promises?

It does not require a callback argument when using Promises. Signup and view all the answers

In the Promise-based code structure, what does 'then' do?

It chains the next function to be executed after the promise resolves. Signup and view all the answers

What is likely to happen if an error occurs in one of the functions chained with 'then'?

The error will be caught and can trigger a failure callback. Signup and view all the answers

What is the purpose of using Promises instead of traditional callbacks in the given examples?

To avoid callback hell and improve readability. Signup and view all the answers

What is the main purpose of using a callback in the function do123?

To handle asynchronous operations Signup and view all the answers

In the Promise-based code structure, what value does the 'resolve' function return?

The result of the asynchronous operation. Signup and view all the answers

What happens if an error occurs in the first function call within do123?

The callback is invoked with the error Signup and view all the answers

What would likely happen if the callback in the traditional method is omitted?

An error will be thrown indicating a missing callback. Signup and view all the answers

What is the purpose of the callback in the do123 function?

To handle errors and return results. Signup and view all the answers

How does the do123 function ensure that the second function waits for the first to complete?

By nesting the second function within the first function's callback Signup and view all the answers

What happens if the first function call in do123 encounters an error?

The callback is called with the error from the first function. Signup and view all the answers

Which statement denotes the correct flow of execution in the Promise-based example provided?

first executes, then second executes only if first resolves. Signup and view all the answers

How does the do123 function handle the results from the second function?

It calls the third function if no error occurs. Signup and view all the answers

What is the final outcome of the do123 function after all three functions are called successfully?

The result of the last function is passed to the callback Signup and view all the answers

What is the significance of checking for 'err1' after the first function call in do123?

To avoid executing subsequent functions in case of an error Signup and view all the answers

What is a major issue with using callbacks in the do123 function?

They create a non-linear code structure. Signup and view all the answers

Which option correctly describes the flow of execution through the do123 function?

first -> if error, end; otherwise, second -> third -> callback Signup and view all the answers

What approach could make the do123 function more manageable?

Converting it to use Promises. Signup and view all the answers

What modification is required for do123 to work with promises instead of callbacks?

Return the result of each function call as a promise Signup and view all the answers

In the do123 function, what will happen if the third function encounters an error?

An error callback will be invoked with the third function's error. Signup and view all the answers

What principle does the final structure of the do123 function demonstrate?

Error-first callback Signup and view all the answers

Which function is executed first when calling do123?

first Signup and view all the answers

What would be the optimal method to improve the flow of asynchronous operations in do123?

Implement async/await syntax. Signup and view all the answers

What does the onReject method handle in a Promise chain?

It is called if the previous Promise rejects or throws an error. Signup and view all the answers

What is the purpose of the finally method in a Promise?

To register a callback that executes regardless of the Promise's result. Signup and view all the answers

What happens when a callback function in the then method does not return a Promise?

The returned value will be wrapped in a resolved Promise. Signup and view all the answers

Which of the following methods creates a Promise object that immediately resolves?

Promise.resolve Signup and view all the answers

What would be printed if the action resolves in the provided code example?

'Error: Action Resolved' Signup and view all the answers

What does the catch method handle in a Promise?

Any errors or rejections in the Promise chain. Signup and view all the answers

What will the finally callback execute in relation to Promise settlements?

Both when the Promise resolves or rejects. Signup and view all the answers

How does Promise.reject behave when called?

It rejects immediately with a specified reason. Signup and view all the answers

What parameter does the `onResolve` function receive?

The resolved value of the previous Promise Signup and view all the answers

In the following code, what will log to the console first: `resolveAfter(500).then(() => resolveAfter(1000))`?

500 Signup and view all the answers

How does the `catch` method function in a Promise?

It registers a callback for handling a rejected Promise Signup and view all the answers

Which of the following correctly uses `then` to handle a resolved Promise?

fetchData().then(handleData); Signup and view all the answers

What do you expect to see in the console if `fetchData()` is called and the Promise resolves successfully?

Fetching data... Signup and view all the answers

Which statement about the `resolveAfter` function is accurate?

It prints the time before resolving the Promise. Signup and view all the answers

What will happen if the Promise returned by `fetchData()` is rejected?

An error will be logged to the console. Signup and view all the answers

In the provided JavaScript functions, what is the purpose of the `setTimeout` method?

To delay the resolution of a Promise Signup and view all the answers

Study Notes

Promises Overview

Promises are a built-in object introduced in ES6
Promises offer a cleaner way to handle asynchronous operations
The callback pattern can become complex when dealing with multiple asynchronous operations
Promises improve the scope of variables and error handling in nested asynchronous operations

Why Use Promises?

Promises provide a more organized structure for asynchronous operations, improving code readability and maintainability.
They simplify the management of asynchronous operations, reducing the risk of errors associated with nested callbacks.
They solve the issue of "callback hell" where multiple nested callbacks make the code hard to follow.

How to Create a Promise

A Promise is created using the new Promise() constructor.
A function (argument to the constructor) with two parameters (resolve, reject) is defined, responsible for handling asynchronous tasks.

Creating a Promise Object

The constructor takes a function as an argument
The function should itself take two arguments: resolve and reject
resolve(value) is called when the asynchronous operation is completed successfully
reject(err) is called when the asynchronous operation fails
Code that depends on the Promise to complete executes within the then() or .catch() blocks

Using the Result of a Promise with then

then(onResolve, onReject) is used to handle the outcome of a Promise
onResolve is called when the preceding Promise resolves successfully, passing the resolved value
onReject is called when the preceding Promise rejects, passing the rejection reason or error

Using the result of a Promise with catch

catch(onReject): used to register a callback function that will handle the result of a rejected Promise
It receives the rejection or error argument
Similar to the then method, also chainable

Using the result of a Promise with finally

finally(oncomplete): used to register a callback to be executed, regardless of whether the Promise resolves successfully or rejects
Allows you to make the code more organized by grouping actions to execute after the Promise execution, not influencing the outcome.

Using Promise.all

Promise.all(promisesArray) accepts an array of Promises
It returns a new Promise that resolves when all Promises in the array resolve.
- The resolved value is an array containing the resolved values of the input promises in the original order.

Using Promise.race

Promise.race(promisesArray) accepts an array of Promises
It returns a new Promise that resolves as soon as any Promise in the array resolves.
- The value for the resolved Promise is the resolved value of the input Promise that resolved first.

Asynchronous Operations with Examples

Real-world examples and scenarios showcasing asynchronous operations.

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Description

This quiz covers the basics of Promises in JavaScript, a crucial feature introduced in ES6 for managing asynchronous operations. Discover how Promises enhance code readability and simplify error handling while avoiding nested callbacks. Test your understanding of creating and using Promise objects effectively.