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!'?

<p>Success! (A)</p> Signup and view all the answers

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

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

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

<p>It returns a Promise object that can resolve or reject. (B)</p> Signup and view all the answers

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

<p>It does not require a callback argument when using Promises. (D)</p> Signup and view all the answers

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

<p>It chains the next function to be executed after the promise resolves. (C)</p> Signup and view all the answers

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

<p>The error will be caught and can trigger a failure callback. (B)</p> Signup and view all the answers

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

<p>To avoid callback hell and improve readability. (D)</p> Signup and view all the answers

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

<p>To handle asynchronous operations (A)</p> Signup and view all the answers

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

<p>The result of the asynchronous operation. (D)</p> Signup and view all the answers

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

<p>The callback is invoked with the error (B)</p> Signup and view all the answers

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

<p>An error will be thrown indicating a missing callback. (A)</p> Signup and view all the answers

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

<p>To handle errors and return results. (C)</p> Signup and view all the answers

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

<p>By nesting the second function within the first function's callback (A)</p> Signup and view all the answers

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

<p>The callback is called with the error from the first function. (B)</p> Signup and view all the answers

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

<p>first executes, then second executes only if first resolves. (C)</p> Signup and view all the answers

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

<p>It calls the third function if no error occurs. (B)</p> Signup and view all the answers

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

<p>The result of the last function is passed to the callback (D)</p> Signup and view all the answers

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

<p>To avoid executing subsequent functions in case of an error (D)</p> Signup and view all the answers

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

<p>They create a non-linear code structure. (C)</p> Signup and view all the answers

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

<p>first -&gt; if error, end; otherwise, second -&gt; third -&gt; callback (C)</p> Signup and view all the answers

What approach could make the do123 function more manageable?

<p>Converting it to use Promises. (B)</p> Signup and view all the answers

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

<p>Return the result of each function call as a promise (C)</p> Signup and view all the answers

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

<p>An error callback will be invoked with the third function's error. (D)</p> Signup and view all the answers

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

<p>Error-first callback (D)</p> Signup and view all the answers

Which function is executed first when calling do123?

<p>first (A)</p> Signup and view all the answers

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

<p>Implement async/await syntax. (A)</p> Signup and view all the answers

What does the onReject method handle in a Promise chain?

<p>It is called if the previous Promise rejects or throws an error. (A)</p> Signup and view all the answers

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

<p>To register a callback that executes regardless of the Promise's result. (A)</p> Signup and view all the answers

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

<p>The returned value will be wrapped in a resolved Promise. (D)</p> Signup and view all the answers

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

<p>Promise.resolve (A)</p> Signup and view all the answers

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

<p>'Error: Action Resolved' (C)</p> Signup and view all the answers

What does the catch method handle in a Promise?

<p>Any errors or rejections in the Promise chain. (B)</p> Signup and view all the answers

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

<p>Both when the Promise resolves or rejects. (D)</p> Signup and view all the answers

How does Promise.reject behave when called?

<p>It rejects immediately with a specified reason. (B)</p> Signup and view all the answers

What parameter does the onResolve function receive?

<p>The resolved value of the previous Promise (B)</p> Signup and view all the answers

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

<p>500 (D)</p> Signup and view all the answers

How does the catch method function in a Promise?

<p>It registers a callback for handling a rejected Promise (C)</p> Signup and view all the answers

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

<p>fetchData().then(handleData); (A)</p> Signup and view all the answers

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

<p>Fetching data... (B)</p> Signup and view all the answers

Which statement about the resolveAfter function is accurate?

<p>It prints the time before resolving the Promise. (A)</p> Signup and view all the answers

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

<p>An error will be logged to the console. (D)</p> Signup and view all the answers

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

<p>To delay the resolution of a Promise (A)</p> Signup and view all the answers

Flashcards

Callback Function

A function that takes an argument and a callback function. The callback function is called with the result of the function.

Promise

A common way to implement asynchronous operations in JavaScript. It allows you to chain multiple operations together in a sequence, so that the next operation only starts once the previous one has finished.

new Promise(func)

A constructor function that takes a single argument, a 'executor' function. The executor function receives two callback functions: 'resolve' and 'reject'.

resolve(result)

A callback function that is called when the Promise operation completes successfully.

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reject(error)

A callback function that is called when the Promise operation fails.

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then(onResolve, onReject)

A method that allows registering callbacks to be triggered when the Promise is resolved or rejected. It returns another Promise, enabling method chaining.

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Promise Callback

A function that is called when a Promise is fulfilled (resolved) or rejected.

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then()

A method used to register callback functions for handling the resolution or rejection of a Promise.

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catch()

A method used to register a callback function for handling the rejection of a Promise.

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new Promise()

A built-in JavaScript function that creates a Promise object.

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onResolve

A function that is called when a Promise is successfully resolved.

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onReject

A function that is called when a Promise is rejected.

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Resolved Value

A value returned by a fulfilled (resolved) Promise.

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Rejected Value

A value returned by a rejected Promise.

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Promise Chaining

The process where multiple Promise operations are executed in a sequence, where each operation starts only after the previous one is finished.

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Callback Hell

A programming pattern where functions are nested within each other, often used to handle asynchronous operations. It leads to code that is hard to read, maintain and reason about.

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Thenable

A function that takes a Promise as input and executes a function when the Promise is resolved successfully.

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Promise Object

An object returned by a Promise constructor, representing the current state of the Promise. It has three states: pending, fulfilled, and rejected.

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Catch Method

A function that handles the rejection of a Promise, providing an error handler if the Promise is rejected.

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Promise.all

A function that allows for multiple Promise operations to be executed in sequence, waiting for each operation to finish before moving to the next.

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Promise.race

A function that allows for multiple Promise operations to be executed in parallel, without waiting for any specific order.

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What is a Promise?

A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.

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States of a Promise

A Promise has three states: Pending, Fulfilled, and Rejected. Initially, it's in the Pending state. When the operation succeeds, it transitions to Fulfilled, and when it fails, it transitions to Rejected.

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Creating a Promise

You create a Promise using the new Promise() constructor, providing an executor function. The executor function takes two arguments: resolve and reject. You call resolve to fulfill the Promise with a value, and reject to reject it with an error.

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Handling a Fulfilled Promise

You use the .then() method to handle the result of a fulfilled Promise. It takes a callback function that receives the resolved value as an argument.

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Handling a Rejected Promise

You use the .catch() method to handle the error of a rejected Promise. It takes a callback function that receives the error as an argument.

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Why use Promises?

Promises allow for cleaner and more readable code, especially when dealing with multiple nested asynchronous operations. They help avoid callback hell.

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Chaining Promises

Promises can be chained together using the .then() method. This means that the result of one Promise can be passed as input to the next Promise.

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Using Promise.all()

You can use the Promise.all() function to wait for multiple promises to resolve and then execute a callback function. This is useful when you need to wait for several asynchronous tasks to complete before moving on.

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finally

A function that is called when a Promise is settled, regardless of whether it resolved or rejected.

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Promise.resolve

A method that creates a Promise object that immediately resolves with the given data.

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Promise wrapping

The return values of callbacks for 'then', 'catch', and 'finally' methods are wrapped as a resolved Promise, if they aren't already.

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finally

A method that allows you to register a callback function to be called when a Promise is settled, regardless of whether it resolves or rejects.

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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.

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