Node.js Promises

Introduction to Promises

Promise States

Pending

Fulfilled

Rejected

Benefits of Using Promises

With Callbacks getUser(id, (err, user) => {
if (err) return handleError(err);
getOrders(user.id, (err, orders) => {
if (err) return handleError(err);

});
});
With Promises getUser(id).then(user => getOrders(user.id)).then(orders => processOrders(orders)).catch(handleError);

Key Advantages:

Easier to compose and chain operations

Built-in support for parallel operations

fs.readFile('file1.txt', (err, data1) => {
if (err) throw err;
fs.readFile('file2.txt', (err, data2) => {
if (err) throw err;
fs.readFile('file3.txt', (err, data3) => {
if (err) throw err;

});
});
});

Creating and Using Promises

Promises can be created using the

Basic Promise Creation

// Create a new Promise const myPromise = new Promise((resolve, reject) => {
// Simulate an async operation (e.g., API call, file read)
setTimeout(() => {
const success = Math.random() > 0.5;
if (success) {
resolve('Operation completed successfully');
} else {
reject(new Error('Operation failed'));
}
}, 1000); // Simulate delay
});
// Using the Promise myPromise.then(result => console.log('Success:', result)).catch(error => console.error('Error:', error.message));

const promise1 = Promise.resolve('First result');
const promise2 = new Promise((resolve) => setTimeout(() => resolve('Second result'), 1000));
const promise3 = fs.readFile('myfile.txt', 'utf8'); // Read local file instead of fetch
Promise.all([promise1, promise2, promise3]).then(results => {
console.log('Results:', results);
// results[0] is from promise1
// results[1] is from promise2
// results[2] is the content of myfile.txt
}).catch(error => {
console.error('Error in one of the promises:', error);
});

Promise Chaining

return new Promise((resolve, reject) => {
// Simulating database call setTimeout(() => {
resolve({ id: userId, name: 'John' });
}, 1000);
});
}
function getUserPosts(user) {
return new Promise((resolve, reject) => {
// Simulating API call setTimeout(() => {
resolve(['Post 1', 'Post 2', 'Post 3']);
}, 1000);
});
}
// Chain the promises getUser(123).then(user => {
console.log('User:', user);
return getUserPosts(user);
}).then(posts => {
console.log('Posts:', posts);
}).catch(error => {
console.error('Error:', error);
});

Promise Methods

Handles fulfillment or rejection catch(onRejected)

Handles rejections finally(onFinally)

Runs after promise settles

Static Methods

Waits for all promises to resolve

Waits for first promise to settle

Waits for all to settle

Utility Methods

Creates a resolved promise

Creates a rejected promise