JavaScript Promises Example

Questions and Answers

What is the expected output when the fetchData function is called?

Failed to fetch data! (correct)

Nothing happens!

Get data successful!

Promise resolved!

How long does the fetchData function wait before it resolves or rejects the promise?

1000 milliseconds

4000 milliseconds

2000 milliseconds (correct)

3000 milliseconds

What role does the 'success' constant play in the fetchData function?

It is used for error handling.

It determines if the promise will resolve or reject. (correct)

It holds the message that will be logged.

It sets the duration for the setTimeout function.

What will be logged to the console if fetchData is called correctly?

<p>The rejection error message.</p> Signup and view all the answers

Which line would cause the 'fetchData' function to execute successfully?

<p>Change 'const success = !true;' to 'const success = true;'</p> Signup and view all the answers

Study Notes

Function Overview

The function fetchData is defined to return a promise.
Promises are used to handle asynchronous operations in JavaScript.

SetTimeout Mechanism

setTimeout is utilized to simulate a delay of 2 seconds before executing the enclosed function.
This mimics fetching data from a network or any asynchronous operation.

Success Condition

A variable success is set to !true, which evaluates to false.
This indicates that the operation (data fetch) will not be successful in this scenario.

Promise Resolution

If success were true, the promise would resolve with an object containing a success message: { data: "Get data successful!" }.
In the current state, when success is false, the promise is rejected with an error message: "Failed to fetch data!".

Handling Promise Results

The fetchData function is called, triggering the promise.
A .then() method is used to handle the fulfilled and rejected states of the promise.
If the promise resolves, the success message would be logged to the console.
Given the current condition, the error message will be logged to the console after 2 seconds.

Console Output

Due to the rejection of the promise, the output after 2 seconds will be: "Failed to fetch data!".

Description

This quiz explores the concept of Promises in JavaScript, focusing on how they handle asynchronous operations. The provided function demonstrates a scenario where data fetching can either succeed or fail, showcasing the use of resolve and reject methods. Test your understanding of how Promises work in JavaScript.