Why asynchronous programming is needed in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to log a message after 2 seconds using setTimeout.

Javascript

setTimeout(() => { console.log('Hello after 2 seconds'); }, [1]);

Drag options to blanks, or click blank then click option'

A2

B2000

C'2 seconds'

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 instead of 2000, which causes the delay to be 2 milliseconds.

The setTimeout function expects the delay in milliseconds. 2000 ms equals 2 seconds.

2fill in blank

medium

Complete the code to fetch data asynchronously using fetch and log the response.

Javascript

fetch('https://api.example.com/data')
  .then(response => response.[1]())
  .then(data => console.log(data));

Drag options to blanks, or click blank then click option'

Ajson

Btext

Csend

Dparse

Attempts:

3 left

💡 Hint

Common Mistakes

Using text() which returns plain text instead of JSON.

The response.json() method parses the response body as JSON.

3fill in blank

hard

Fix the error in the async function to await the fetch call.

Javascript

async function getData() {
  const response = [1] fetch('https://api.example.com/data');
  const data = await response.json();
  console.log(data);
}

Drag options to blanks, or click blank then click option'

Aasync

Breturn

Cthen

Dawait

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting await causes response to be a promise, not the actual response.

The await keyword pauses the function until the fetch promise resolves.

4fill in blank

hard

Fill both blanks to create a promise that resolves after 1 second.

Javascript

const wait = () => new Promise(([1], [2]) => {
  setTimeout(resolve, 1000);
});

Drag options to blanks, or click blank then click option'

Aresolve

Breject

Ccallback

Ddone

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names like callback or done.

A promise executor function receives two arguments: resolve and reject.

5fill in blank

hard

Fill all three blanks to handle errors in an async function using try-catch.

Javascript

async function fetchData() {
  try {
    const response = await fetch([1]);
    const data = await response.[2]();
    console.log(data);
  } catch ([3]) {
    console.error('Error:', error);
  }
}

Drag options to blanks, or click blank then click option'

A'https://api.example.com/data'

Bjson

Cerror

Dresponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names or missing quotes around the URL.

The fetch function needs the URL string, response.json() parses JSON, and the catch block catches the error variable.

Practice

1. Why do we need asynchronous programming in JavaScript?

easy

A. To write shorter code

B. To make the code run slower

C. To avoid using variables

D. To prevent the program from freezing during slow tasks

Solution

1. Step 1: Understand what happens during slow tasks

   Slow tasks like loading files or waiting for internet can block the program if done one by one.

2. Step 2: Role of asynchronous programming

   Asynchronous programming lets the program continue running other tasks without waiting for slow ones to finish.

3. Final Answer:

   To prevent the program from freezing during slow tasks -> Option D

4. Quick Check:

   Asynchronous = prevent freezing [OK]

Hint: Asynchronous means not waiting, so no freezing [OK]

Common Mistakes:

• Thinking asynchronous makes code slower
• Confusing asynchronous with shorter code
• Believing asynchronous avoids variables

2. Which of the following is the correct way to start an asynchronous task in JavaScript?

easy

A. const result = pause('url');

B. const result = wait('url');

C. const result = fetch('url');

D. const result = sleep('url');

Solution

1. Step 1: Identify the correct asynchronous function

   In JavaScript, fetch() is used to start an asynchronous request to get data from a URL.

2. Step 2: Check other options

   Functions like wait(), sleep(), and pause() do not exist in JavaScript for asynchronous tasks.

3. Final Answer:

   const result = fetch('url'); -> Option C

4. Quick Check:

   fetch() starts async tasks [OK]

Hint: Remember fetch() is the async web request function [OK]

Common Mistakes:

• Using non-existent functions like wait or sleep
• Confusing synchronous and asynchronous functions
• Assuming all functions pause execution

3. What will this code output?

console.log('Start');
setTimeout(() => console.log('Middle'), 0);
console.log('End');

medium

A. Start\nMiddle\nEnd

B. Start\nEnd\nMiddle

C. Middle\nStart\nEnd

D. End\nStart\nMiddle

Solution

1. Step 1: Understand synchronous and asynchronous order

   console.log('Start') runs immediately, then setTimeout schedules 'Middle' to run later, then console.log('End') runs immediately.

2. Step 2: Determine output order

   'Start' prints first, 'End' prints second, and 'Middle' prints last after the current code finishes.

3. Final Answer:

   Start\nEnd\nMiddle -> Option B

4. Quick Check:

   setTimeout delays output [OK]

Hint: setTimeout runs after current code, so last output [OK]

Common Mistakes:

• Assuming setTimeout with 0 delay runs immediately
• Thinking code runs top to bottom without delay
• Ignoring asynchronous nature of setTimeout

4. Find the error in this code that tries to fetch data asynchronously:

const data = fetch('https://api.example.com/data');
console.log(data);

medium

A. fetch returns a Promise, so data is not the actual data yet

B. fetch is a synchronous function, so no error

C. Missing semicolon after fetch call

D. URL is incorrect syntax

Solution

1. Step 1: Understand what fetch returns

   fetch returns a Promise object immediately, not the actual data.

2. Step 2: Why console.log shows Promise

   Logging data shows a Promise because the data is not yet received; you must wait for it asynchronously.

3. Final Answer:

   fetch returns a Promise, so data is not the actual data yet -> Option A

4. Quick Check:

   fetch() returns Promise, not data [OK]

Hint: fetch returns Promise, not data directly [OK]

Common Mistakes:

• Thinking fetch returns data immediately
• Ignoring Promise nature of fetch
• Assuming missing semicolon causes error

5. You want to load user data from a server and then show a welcome message. Which approach correctly uses asynchronous programming to do this?

hard

A. Use fetch() with .then() to wait for data before showing message

B. Call fetch() and immediately show the message without waiting

C. Use a while loop to wait until fetch finishes

D. Write synchronous code that blocks until data loads

Solution

1. Step 1: Understand asynchronous data loading

   fetch() returns a Promise, so you must wait for it to finish before using the data.

2. Step 2: Using .then() to handle Promise

   Using .then() lets you run code after data loads, so the welcome message shows with correct data.

3. Step 3: Why other options fail

   Showing message immediately ignores data; while loop blocks UI; synchronous blocking is bad for user experience.

4. Final Answer:

   Use fetch() with .then() to wait for data before showing message -> Option A

5. Quick Check:

   Use .then() to handle async data [OK]

Hint: Use .then() to wait for async data before next step [OK]

Common Mistakes:

• Showing message before data loads
• Blocking UI with loops
• Trying to write synchronous code for async tasks