Why promises are used in Javascript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Promise.

Javascript

const myPromise = new [1]((resolve, reject) => {
  resolve('Done');
});

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

APromise

BFunction

CObject

DArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'promise' instead of 'Promise'.

Using 'Function' instead of 'Promise'.

Promises in JavaScript are created using the Promise constructor.

2fill in blank

medium

Complete the code to handle a resolved Promise.

Javascript

myPromise.[1](result => {
  console.log(result);
});

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

Acatch

Bthen

Cfinally

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'catch' to handle success.

Using 'resolve' as a method.

The then method is used to handle the result when a Promise is resolved.

3fill in blank

hard

Fix the error in the code to handle Promise rejection.

Javascript

myPromise.then(result => {
  console.log(result);
}).[1](error => {
  console.error(error);
});

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

Afinally

Bthen

Ccatch

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'then' to catch errors.

Using 'finally' to catch errors.

The catch method is used to handle errors when a Promise is rejected.

4fill in blank

hard

Fill both blanks to create a Promise that rejects on error.

Javascript

const errorPromise = new Promise(([1], [2]) => {
  [1]('Success');
  [2]('Error occurred');
});

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

Aresolve

Breject

Cresult

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping resolve and reject.

Using variable names instead of function names.

The first parameter is resolve to handle success, the second is reject to handle errors.

5fill in blank

hard

Fill all three blanks to create a Promise and handle both success and failure.

Javascript

const promise = new Promise(([1], [2]) => {
  if(true) {
    [1]('Done');
  } else {
    [2]('Failed');
  }
});
promise.[3](message => console.log(message)).catch(error => console.error(error));

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

Aresolve

Breject

Cthen

Dfinally

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'finally' instead of 'then'.

Mixing up resolve and reject.

Use resolve and reject in the Promise constructor, and then to handle success.

Practice

1. Why are promises used in JavaScript?
Promise helps with:

easy

A. Storing data permanently

B. Making the program run slower

C. Writing code without any functions

D. Handling tasks that take time without stopping the program

Solution

1. Step 1: Understand what promises do

   Promises allow JavaScript to handle tasks like loading data without freezing the program.

2. Step 2: Identify the correct purpose

   Promises help manage asynchronous tasks so the program keeps running smoothly.

3. Final Answer:

   Handling tasks that take time without stopping the program -> Option D

4. Quick Check:

   Promises = handle async tasks without blocking [OK]

Hint: Promises handle slow tasks without freezing code [OK]

Common Mistakes:

• Thinking promises slow down the program
• Confusing promises with data storage
• Believing promises remove the need for functions

2. Which of the following is the correct way to create a promise in JavaScript?

easy

A. let p = new Promise((resolve, reject) => { /* code */ });

B. let p = Promise.create(() => { /* code */ });

C. let p = promise(() => { /* code */ });

D. let p = new promise { /* code */ };

Solution

1. Step 1: Recall correct Promise syntax

   Promises are created with new Promise and a function with resolve and reject parameters.

2. Step 2: Check each option

   Only let p = new Promise((resolve, reject) => { /* code */ }); uses the correct syntax with new Promise and arrow function.

3. Final Answer:

   let p = new Promise((resolve, reject) => { /* code */ }); -> Option A

4. Quick Check:

   Correct Promise syntax = new Promise(...) [OK]

Hint: Use 'new Promise' with resolve and reject functions [OK]

Common Mistakes:

• Using lowercase 'promise' instead of 'Promise'
• Trying to call Promise.create which doesn't exist
• Missing parentheses or arrow function syntax

3. What will be the output of this code?

const p = new Promise((resolve) => {
  setTimeout(() => resolve('Done'), 100);
});
p.then(result => console.log(result));
console.log('Start');

medium

A. StartDone

B. DoneStart

C. DoneDone

D. StartStart

Solution

1. Step 1: Understand asynchronous behavior

   The setTimeout delays the resolve call by 100ms, so console.log('Start') runs first.

2. Step 2: Trace the output order

   First, 'Start' prints immediately, then after 100ms, 'Done' prints from the promise.

3. Final Answer:

   Start
   Done -> Option A

4. Quick Check:

   Async delay means 'Start' before 'Done' [OK]

Hint: setTimeout delays promise resolution, so logs appear after immediate code [OK]

Common Mistakes:

• Assuming promise resolves immediately
• Thinking 'Done' prints before 'Start'
• Ignoring asynchronous nature of setTimeout

4. Find the error in this promise code:

const p = new Promise((resolve, reject) => {
  resolve('Success');
  reject('Error');
});
p.then(result => console.log(result))
 .catch(error => console.log(error));

medium

A. Promise must call reject first

B. Calling reject after resolve has no effect

C. Missing semicolon after resolve

D. Then and catch cannot be chained

Solution

1. Step 1: Understand promise state changes

   Once a promise is resolved or rejected, its state is final and cannot change.

2. Step 2: Analyze the code flow

   The promise calls resolve('Success') first, so the later reject('Error') is ignored.

3. Final Answer:

   Calling reject after resolve has no effect -> Option B

4. Quick Check:

   Promise settles once; later calls ignored [OK]

Hint: Promise settles once; later resolve/reject calls ignored [OK]

Common Mistakes:

• Thinking reject overrides resolve if called later
• Believing then and catch can't be chained
• Confusing semicolon placement with logic errors

5. You want to fetch user data and then fetch posts for that user. Which approach best uses promises to handle this sequence?

hard

A. Use setTimeout to delay fetchPosts() after fetchUser()

B. Call fetchUser() and fetchPosts() separately without waiting

C. Call fetchUser(), then inside its .then(), call fetchPosts() and return that promise

D. Call fetchPosts() first, then fetchUser() inside its .then()

Solution

1. Step 1: Understand promise chaining for dependent tasks

   When one task depends on another, use .then() to wait for the first before starting the second.

2. Step 2: Identify correct chaining

   Call fetchUser(), then inside its .then(), call fetchPosts() and return that promise correctly calls fetchUser(), then in its .then(), calls fetchPosts() and returns its promise to chain properly.

3. Final Answer:

   Call fetchUser(), then inside its .then(), call fetchPosts() and return that promise -> Option C

4. Quick Check:

   Promise chaining = dependent tasks in sequence [OK]

Hint: Chain promises with then() for dependent async tasks [OK]

Common Mistakes:

• Running dependent tasks in parallel without waiting
• Using setTimeout instead of promise chaining
• Calling tasks in wrong order