Async function syntax in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of an async function returning a value

What is the output of this code when run in a modern JavaScript environment?

Javascript

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

greet().then(console.log);

A Hello!

B Promise { 'Hello!' }

C SyntaxError

D undefined

💡 Hint

Remember that async functions always return a Promise, but .then unwraps the value.

The async function greet returns a Promise that resolves to 'Hello!'. Using .then(console.log) prints the resolved value 'Hello!'.

❓ Predict Output

intermediate

Output of an async function with await

What will this code output to the console?

Javascript

async function getNumber() {
  const num = await Promise.resolve(42);
  return num;
}

getNumber().then(console.log);

A 42

B Promise { 42 }

C undefined

D TypeError

💡 Hint

The await keyword unwraps the Promise's resolved value.

The await pauses until the Promise resolves to 42, then returns 42. The .then logs 42.

❓ Predict Output

advanced

Error when missing async keyword

What error does this code produce?

Javascript

function fetchData() {
  await Promise.resolve('data');
}

fetchData();

A ReferenceError

B No error, runs fine

C TypeError

D SyntaxError

💡 Hint

await can only be used inside async functions.

Using await outside an async function causes a SyntaxError.

❓ Predict Output

advanced

Return value of an async arrow function

What is the output of this code?

Javascript

const asyncArrow = async () => 10;

asyncArrow().then(console.log);

A SyntaxError

B Promise { 10 }

C 10

D undefined

💡 Hint

Async arrow functions return Promises resolved with the returned value.

The async arrow function returns a Promise resolved with 10. The .then logs 10.

🧠 Conceptual

expert

Behavior of async function without await

What is the value of variable result after running this code?

Javascript

async function noAwait() {
  return 5;
}

const result = noAwait();

A 5

B Promise { 5 }

C undefined

D TypeError

💡 Hint

Async functions always return a Promise, even if no await is used.

The function noAwait returns a Promise resolved with 5. So result is a Promise object.

Practice

1. What does the async keyword do when placed before a function in JavaScript?

easy

A. It converts the function into a synchronous blocking call.

B. It makes the function run faster by using multiple CPU cores.

C. It automatically retries the function if it fails.

D. It makes the function return a promise and allows use of await inside it.

Solution

1. Step 1: Understand the role of async

   The async keyword marks a function to always return a promise.

2. Step 2: Recognize the effect on await

   Inside an async function, you can use await to pause until a promise resolves.

3. Final Answer:

   It makes the function return a promise and allows use of await inside it. -> Option D

4. Quick Check:

   async = returns promise + allows await [OK]

Hint: Async means function returns promise and can use await inside [OK]

Common Mistakes:

• Thinking async makes function run faster
• Confusing async with synchronous blocking
• Believing async retries failed calls automatically

2. Which of the following is the correct syntax to declare an async function named fetchData?

easy

A. async function fetchData() { }

B. function async fetchData() { }

C. function fetchData async() { }

D. async fetchData function() { }

Solution

1. Step 1: Recall async function declaration syntax

   The correct syntax places async before the function keyword.

2. Step 2: Check each option's order

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

3. Final Answer:

   async function fetchData() { } -> Option A

4. Quick Check:

   async before function keyword = correct syntax [OK]

Hint: Put async before function keyword for correct syntax [OK]

Common Mistakes:

• Placing async after function name
• Writing async inside parentheses
• Mixing order of async and function keywords

3. What will be the output of this code?

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

greet().then(console.log);

medium

A. Promise { 'Hello' }

B. Hello

C. undefined

D. SyntaxError

Solution

1. Step 1: Understand async function return value

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

2. Step 2: Analyze the .then usage

   The then method receives the resolved value 'Hello' and logs it.

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   Async function returns promise, .then logs resolved value [OK]

Hint: Async returns promise; .then logs resolved value [OK]

Common Mistakes:

• Expecting direct string without .then
• Confusing promise object with resolved value
• Thinking async function returns undefined

4. Identify the error in this async function:

async function load() {
  const data = await fetch('url');
  return data.json;
}

medium

A. fetch should not be awaited

B. Cannot use await inside async function

C. Missing parentheses after json to call the method

D. async keyword is misplaced

Solution

1. Step 1: Check usage of await and fetch

   Using await fetch('url') is correct inside async function.

2. Step 2: Inspect return statement

   data.json is a function and needs parentheses: data.json().

3. Final Answer:

   Missing parentheses after json to call the method -> Option C

4. Quick Check:

   Call json() method with parentheses [OK]

Hint: Remember to call json() with parentheses after await fetch [OK]

Common Mistakes:

• Forgetting parentheses on json method
• Thinking await can't be used inside async
• Misplacing async keyword

5. You want to write an async function processData that fetches JSON from a URL and returns the value of the name property. Which code correctly does this?

hard

A. async function processData(url) {
const response = await fetch(url);
const data = await response.json();
return data.name;
}

B. async function processData(url) {
const response = fetch(url);
const data = response.json();
return data.name;
}

C. function processData(url) {
const response = await fetch(url);
const data = await response.json();
return data.name;
}

D. async function processData(url) {
const response = await fetch(url);
const data = response.json;
return data.name;
}

Solution

1. Step 1: Confirm async function and await usage

   async function processData(url) { const response = await fetch(url); const data = await response.json(); return data.name; } correctly uses async and awaits both fetch and response.json().

2. Step 2: Check return of name property

   async function processData(url) { const response = await fetch(url); const data = await response.json(); return data.name; } returns data.name after parsing JSON, which is correct.

3. Final Answer:

   async function processData(url) { const response = await fetch(url); const data = await response.json(); return data.name; } -> Option A

4. Quick Check:

   Await fetch and json(), then return property [OK]

Hint: Await fetch and json(), then return needed property [OK]

Common Mistakes:

• Not awaiting response.json()
• Missing async keyword
• Returning json method without calling it