Understanding Promises in Node.js

Overview

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. Promises provide a cleaner alternative to callbacks for handling asynchronous code.

Creating a Promise

Here‘s how to create a basic Promise:

const myPromise = new Promise((resolve, reject) => {
  const success = true; // Simulate success or failure

  if (success) {
    resolve('Operation was successful!');
  } else {
    reject('Operation failed.');
  }
});

In this example, the Promise will either resolve or reject based on the simulated success condition.

Using Promises

To handle the result of a Promise, you can use the .then() and .catch() methods:

myPromise
  .then(result => {
    console.log(result); // Output: Operation was successful!
  })
  .catch(error => {
    console.error(error); // Handle error
  });

This approach allows you to handle successful outcomes and errors separately.

Chaining Promises

Promises can be chained to perform multiple asynchronous operations in sequence:

function fetchData() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve('Data fetched');
    }, 1000);
  });
}

fetchData()
  .then(data => {
    console.log(data); // Output: Data fetched
    return 'Processed data';
  })
  .then(processedData => {
    console.log(processedData); // Output: Processed data
  })
  .catch(error => {
    console.error(error); // Handle any error
  });

In this example, each .then() receives the result of the previous Promise, allowing for smooth chaining.

Error Handling with Promises

It‘s important to handle errors in a chain of Promises. Any error that occurs in a Promise will skip to the nearest .catch():

function fetchDataWithError() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject('Error fetching data');
    }, 1000);
  });
}

fetchDataWithError()
  .then(data => {
    console.log(data);
  })
  .catch(error => {
    console.error(error); // Output: Error fetching data
  });