Async function syntax in Javascript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an async function named fetchData.

Javascript

async function [1]() {
  return 'data';
}

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

AgetData

BfetchData

CloadData

DretrieveData

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name than requested.

Forgetting to write the function name after async function.

The function name should be fetchData as requested.

2fill in blank

medium

Complete the code to declare an async arrow function assigned to fetchData.

Javascript

const fetchData = [1] () => {
  return 'data';
};

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

Aasync

Bawait

Cfunction

Dreturn

Attempts:

3 left

💡 Hint

Common Mistakes

Placing await instead of async before the arrow function.

Using function keyword inside arrow function declaration.

The async keyword should come before the parentheses in an arrow function.

3fill in blank

hard

Fix the error in the async function declaration by completing the code.

Javascript

async [1] fetchData() {
  return 'data';
}

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

Afunction

Basync

Cconst

Dlet

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the function keyword after async.

Using variable declaration keywords like const or let incorrectly.

The correct syntax for declaring an async function is async function fetchData().

4fill in blank

hard

Complete the code to create an async function expression assigned to fetchData.

Javascript

const fetchData = [1] function () {
  return 'data';
};

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

Aasync

Bawait

D()

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses or other tokens between function and parentheses incorrectly.

Using await instead of async before function.

The async keyword comes before the function keyword, and there is a space (empty string) between function and parentheses.

5fill in blank

hard

Fill all three blanks to create an async method inside an object named api.

Javascript

const api = {
  [1]: async function() {
    return [2];
  },
  [3]() {
    return 'sync';
  }
};

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

AfetchData

B'async data'

CsyncMethod

DasyncMethod

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing method names or forgetting to use async keyword.

Returning wrong values or missing quotes around strings.

The async method is named asyncMethod, it returns 'async data', and the synchronous method is named syncMethod.

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