Promises for cleaner async in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Promise in JavaScript?

A Promise is an object that represents the eventual completion or failure of an asynchronous operation. It helps write cleaner code by avoiding deeply nested callbacks.

beginner

What are the three states of a Promise?

A Promise can be in one of three states: pending (initial state), fulfilled (operation completed successfully), or rejected (operation failed).

beginner

How does the then() method work with Promises?

The then() method is used to specify what to do when a Promise is fulfilled. It takes a callback function that runs with the resolved value.

beginner

What is the purpose of the catch() method in Promises?

The catch() method handles errors or rejections from a Promise. It runs a callback if the Promise is rejected, helping to manage failures cleanly.

intermediate

How do Promises help make asynchronous code cleaner compared to callbacks?

Promises avoid "callback hell" by chaining then() calls instead of nesting callbacks. This makes the code easier to read and maintain.

Which Promise state means the operation is still running?

A Rejected

B Pending

C Fulfilled

D Resolved

A Promise is 'pending' while the asynchronous operation is still in progress.

Which method is used to handle a successful Promise result?

A then()

B catch()

C finally()

D resolve()

The then() method runs a callback when the Promise is fulfilled successfully.

What does the catch() method do in a Promise chain?

A Returns the Promise value

B Starts the Promise

C Handles errors or rejections

D Cancels the Promise

catch() handles errors or rejected Promises to keep code clean and avoid crashes.

Which of these is NOT a Promise state?

A Resolved

B Rejected

C Fulfilled

D Pending

'Resolved' is not an official Promise state; the correct states are pending, fulfilled, and rejected.

Why are Promises preferred over nested callbacks?

A They run faster

B They avoid using functions

C They use less memory

D They make code easier to read and maintain

Promises allow chaining which avoids deeply nested callbacks, making code cleaner and easier to follow.

Explain how Promises improve asynchronous code compared to callbacks.

Think about how you handle success and errors without nesting functions inside each other.

Describe the lifecycle of a Promise from creation to completion.

Imagine waiting for a package delivery: waiting, delivered, or failed.

Practice

1. What is the main purpose of using Promises in Node.js?

easy

A. To store data permanently on disk

B. To make the program run faster by using multiple CPUs

C. To write synchronous code only

D. To handle asynchronous tasks without freezing the program

Solution

1. Step 1: Understand asynchronous tasks

   Asynchronous tasks take time and can block the program if not handled properly.

2. Step 2: Role of Promises

   Promises allow handling these tasks without freezing the program by running code after the task finishes.

3. Final Answer:

   To handle asynchronous tasks without freezing the program -> Option D

4. Quick Check:

   Promises manage async tasks = A [OK]

Hint: Promises help avoid freezing during slow tasks [OK]

Common Mistakes:

• Thinking Promises speed up code execution
• Confusing Promises with synchronous code
• Believing Promises store data permanently

2. Which of the following is the correct way to create a Promise in Node.js?

easy

A. const p = Promise(() => { resolve(); });

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

C. const p = new Promise(resolve, reject);

D. const p = Promise.new((resolve, reject) => { });

Solution

1. Step 1: Check Promise constructor syntax

   The Promise constructor requires a function with two parameters: resolve and reject.

2. Step 2: Validate each option

   const p = new Promise((resolve, reject) => { /* code */ }); correctly uses new Promise with a function taking resolve and reject.

3. Final Answer:

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

4. Quick Check:

   Correct Promise syntax = B [OK]

Hint: Use 'new Promise' with (resolve, reject) function [OK]

Common Mistakes:

• Omitting 'new' keyword
• Passing resolve and reject outside a function
• Using incorrect Promise constructor syntax

3. What will the following code output?

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

medium

A. Start only

B. Done\nStart

C. Start\nDone

D. Done only

Solution

1. Step 1: Understand asynchronous setTimeout

   The setTimeout delays resolve by 100ms, so 'Done' logs after delay.

2. Step 2: Order of console logs

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

3. Final Answer:

   Start\nDone -> Option C

4. Quick Check:

   Async delay means 'Start' first, then 'Done' [OK]

Hint: Immediate logs appear before delayed Promise results [OK]

Common Mistakes:

• Assuming Promise resolves immediately
• Expecting 'Done' before 'Start'
• Ignoring asynchronous behavior of setTimeout

4. Identify 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. Calling reject after resolve has no effect

B. Missing catch block for errors

C. Promise constructor missing 'new' keyword

D. resolve and reject parameters are reversed

Solution

1. Step 1: Understand Promise state changes

   Once a Promise is resolved or rejected, further calls to resolve or reject are ignored.

2. Step 2: Analyze code behavior

   The code calls resolve first, so reject after that does nothing.

3. Final Answer:

   Calling reject after resolve has no effect -> Option A

4. Quick Check:

   Promise settles once; later calls ignored [OK]

Hint: Promise settles once; ignore calls after resolve/reject [OK]

Common Mistakes:

• Expecting both resolve and reject to run
• Forgetting Promise settles only once
• Confusing order of resolve and reject calls

5. You want to run two async tasks one after another using Promises. Which code correctly chains them to run sequentially?

hard

A. task1().then(() => task2()).then(result => console.log(result));

B. Promise.all([task1(), task2()]).then(results => console.log(results));

C. task1(); task2(); console.log('Done');

D. task1().catch(() => task2()).then(result => console.log(result));

Solution

1. Step 1: Understand sequential chaining

   To run tasks one after another, call the second in the first's .then() callback.

2. Step 2: Analyze options

   task1().then(() => task2()).then(result => console.log(result)); chains task2 after task1 completes, ensuring order.

3. Final Answer:

   task1().then(() => task2()).then(result => console.log(result)); -> Option A

4. Quick Check:

   Chain with .then() for sequential async tasks [OK]

Hint: Use .then() chaining to run tasks one after another [OK]

Common Mistakes:

• Using Promise.all for sequential tasks (runs parallel)
• Calling tasks without chaining (runs parallel)
• Misusing catch to run second task