Promises for cleaner async in Node.js

Introduction

Promises help you handle tasks that take time, like loading data, without freezing your program. They make your code easier to read and manage when working with things that happen later.

When you want to load data from the internet without stopping other parts of your program.

When you need to wait for a file to be read before continuing.

When you want to run several tasks one after another, but only after the previous one finishes.

When you want to handle errors from asynchronous tasks in a clean way.

When you want to avoid deeply nested callbacks that are hard to read.

Syntax

Node.js

new Promise((resolve, reject) => {
  // Do some async work
  if (success) {
    resolve(result); // Task finished successfully
  } else {
    reject(error); // Task failed
  }
});

resolve is called when the task finishes well.

reject is called when something goes wrong.

Examples

This promise waits 1 second, then says it is done.

Node.js

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Done!');
  }, 1000);
});

Here we say what to do when the promise finishes or fails.

Node.js

myPromise.then(result => {
  console.log(result);
}).catch(error => {
  console.error(error);
});

Using promises to get data from the internet and handle errors.

Node.js

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error:', error));

Sample Program

This program waits 1.5 seconds, prints a message, then waits 1 more second and prints another message. It uses promises to handle the waiting cleanly.

Node.js

function wait(ms) {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve(`Waited ${ms} milliseconds`);
    }, ms);
  });
}

wait(1500)
  .then(message => {
    console.log(message);
    return wait(1000);
  })
  .then(message => {
    console.log(message);
  })
  .catch(error => {
    console.error('Error:', error);
  });

Important Notes

Promises let you chain tasks with .then() for better flow.

Always add .catch() to handle errors and avoid crashes.

Promises make your code easier to read compared to nested callbacks.

Summary

Promises help manage tasks that take time without freezing your program.

Use resolve and reject to signal success or failure.

Chain .then() and .catch() to handle results and errors cleanly.

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