Async function syntax in Javascript - Mini Project: Build & Apply

Async function syntax

📖 Scenario: You want to fetch user data from a server and then show a greeting message. Using asynchronous functions helps you wait for the data without freezing the page.

🎯 Goal: Build a simple async function that waits for user data and then prints a greeting message.

📋 What You'll Learn

Create an async function named getUser that returns a user object after a delay.

Create a variable userId with the value 1.

Use await inside an async function to get the user data.

Print the greeting message using the user's name.

💡 Why This Matters

🌍 Real World

Async functions let web pages get data from servers without freezing or waiting, making apps smooth and fast.

💼 Career

Understanding async functions is key for web developers to handle data fetching, user interactions, and background tasks efficiently.

1

Create the async function getUser

Write an async function called getUser that returns a promise resolving to the object { id: 1, name: 'Alice' } after 1 second using setTimeout.

Javascript

// Your code here

Hint

Use async function and return a new Promise that resolves after 1 second.

2

Create the userId variable

Create a variable called userId and set it to the number 1.

Javascript

async function getUser() {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve({ id: 1, name: 'Alice' });
        }, 1000);
    });
}

// Your code here

Hint

Use const userId = 1; to create the variable.

3

Use await to get user data

Write an async function called showGreeting. Inside it, use await getUser() to get the user object and store it in a variable called user.

Javascript

async function getUser() {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve({ id: 1, name: 'Alice' });
        }, 1000);
    });
}

const userId = 1;

// Your code here

Hint

Remember to mark showGreeting as async to use await inside it.

4

Print the greeting message

Inside the showGreeting function, add a console.log statement that prints `Hello, ${user.name}!`. Then call showGreeting() to run it.

Javascript

async function getUser() {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve({ id: 1, name: 'Alice' });
        }, 1000);
    });
}

const userId = 1;

async function showGreeting() {
    const user = await getUser();
    // Your code here
}

// Call showGreeting() below
// Your code here

Hint

Use console.log(`Hello, ${user.name}!`) and call showGreeting().

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