Why async and await are needed in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a function that returns a promise.

Javascript

function fetchData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve('Data loaded'), [1]);
  });
}

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

A'1000'

BsetTimeout

C1000

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a number for delay time.

The number 1000 means 1000 milliseconds (1 second) delay before resolving the promise.

2fill in blank

medium

Complete the code to use async function syntax.

Javascript

async function [1]() {
  const result = await fetchData();
  console.log(result);
}

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

AloadData

BgetData

CfetchData

Dload

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name as the promise function, which can cause confusion.

The function name can be anything, but here we choose 'loadData' to show an async function that waits for data.

3fill in blank

hard

Fix the error in the code by adding the missing keyword.

Javascript

async function load() {
  const data = [1] fetchData();
  console.log(data);
}

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

Aawait

Basync

Cthen

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'await' outside an async function causes an error.

The 'await' keyword is needed to wait for the promise to resolve inside an async function.

4fill in blank

hard

Fill both blanks to correctly handle the promise result.

Javascript

async function getData() {
  try {
    const result = [1] fetchData();
    console.log([2]);
  } catch (error) {
    console.error(error);
  }
}

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

Aawait

Bresult

Cdata

DfetchData

Attempts:

3 left

💡 Hint

Common Mistakes

Logging the function name instead of the result variable.

Use 'await' to wait for the promise, and then log the variable 'result' that holds the data.

5fill in blank

hard

Fill all three blanks to create an async function that fetches data and handles errors.

Javascript

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

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

Adata

Bawait

DfetchData

Attempts:

3 left

💡 Hint

Common Mistakes

Using the function name instead of the awaited result variable.

We declare a variable 'data', use 'await' to wait for the promise, and then log 'data'.

Practice

1. Why do we use async and await in JavaScript?

easy

A. To make the code run faster by skipping steps

B. To handle slow tasks without freezing the program

C. To write code that only works in old browsers

D. To stop the program completely until a task finishes

Solution

1. Step 1: Understand the problem with slow tasks

   Slow tasks like fetching data can freeze the program if handled synchronously.

2. Step 2: Role of async and await

   Async and await let the program wait for slow tasks without freezing, keeping it smooth.

3. Final Answer:

   To handle slow tasks without freezing the program -> Option B

4. Quick Check:

   Async/await = smooth waiting [OK]

Hint: Async/await = smooth waiting without freezing [OK]

Common Mistakes:

• Thinking async/await makes code faster
• Believing async/await stops the whole program
• Confusing async/await with old browser code

2. Which of the following is the correct way to declare an async function in JavaScript?

easy

A. function async fetchData() {}

B. function fetchData() async {}

C. async function fetchData() {}

D. function fetchData async() {}

Solution

1. Step 1: Recall async function syntax

   The correct syntax places async before the function keyword.

2. Step 2: Check each option

   Only async function fetchData() {} correctly uses async function fetchData() {}.

3. Final Answer:

   async function fetchData() {} -> Option C

4. Quick Check:

   Async before function keyword [OK]

Hint: Async keyword goes before function name [OK]

Common Mistakes:

• Placing async after function name
• Adding async inside function parentheses
• Writing async after function body

3. What will be the output of this code?

async function greet() {
  return 'Hello';
}

greet().then(console.log);

medium

A. 'Hello'

B. Promise { 'Hello' }

C. undefined

D. Error: greet is not a function

Solution

1. Step 1: Understand async function return

   An async function returns a Promise that resolves to the returned value.

2. Step 2: Using .then to get value

   The .then(console.log) prints the resolved value 'Hello'.

3. Final Answer:

   'Hello' -> Option A

4. Quick Check:

   Async function returns Promise, .then logs value [OK]

Hint: Async returns Promise; .then gets resolved value [OK]

Common Mistakes:

• Expecting direct string without .then
• Confusing Promise object with string
• Thinking async function returns undefined

4. Find the error in this code:

async function fetchData() {
  const data = await fetch('https://api.example.com');
  return data.json();
}

const result = fetchData();
console.log(result);

medium

A. data.json() is not a function

B. fetchData() should not be async

C. Cannot use await inside async function

D. Missing await before fetchData() call

Solution

1. Step 1: Check fetchData() call

   fetchData() returns a Promise, so logging it directly shows a Promise object.

2. Step 2: Fix by awaiting fetchData()

   Adding await before fetchData() inside an async context gets the resolved data.

3. Final Answer:

   Missing await before fetchData() call -> Option D

4. Quick Check:

   Await needed to get Promise result [OK]

Hint: Await async calls to get actual result [OK]

Common Mistakes:

• Logging Promise instead of awaited value
• Removing async keyword incorrectly
• Misunderstanding data.json() usage

5. You want to fetch user data and then fetch posts by that user. Which code correctly uses async/await to do this step-by-step without freezing the program?

function getUser() {
  return fetch('/user').then(res => res.json());
}

function getPosts(userId) {
  return fetch(`/posts?user=${userId}`).then(res => res.json());
}

// Fill in the blanks
async function loadData() {
  const user = _____ getUser();
  const posts = _____ getPosts(user.id);
  console.log(posts);
}

hard

A. await, await

B. await, then

C. then, await

D. then, then

Solution

1. Step 1: Understand async function usage

   Inside async functions, use await to wait for Promises step-by-step.

2. Step 2: Apply await to both calls

   Both getUser() and getPosts() return Promises, so both need await.

3. Final Answer:

   await, await -> Option A

4. Quick Check:

   Use await for sequential async calls [OK]

Hint: Use await for each async call inside async function [OK]

Common Mistakes:

• Mixing then and await inside async function
• Forgetting await causing Promise logs
• Using then without returning inside async