JavaScript Promises

Asynchronous Operations:

• Promises represent the eventual completion or failure of an asynchronous operation, allowing better management of asynchronous code execution.

States:

• Promises can be in one of three states: pending, fulfilled, or rejected.
• Once a promise settles (either fulfilled or rejected), it is considered settled and will not change its state again.

Chaining:

• Promises support chaining, allowing sequential execution of asynchronous tasks.
• The result of one promise can be passed to the next in the chain.

Error Handling:

• Promises provide robust error handling through the catch() method.
• Errors in the promise chain propagate to the nearest catch() method for centralized error handling.

Example: Creating a Promise

const fetchData = new Promise((resolve, reject) => {
  setTimeout(() => {
    const data = 'This is some data';
    // Simulate success
    resolve(data);
    // Simulate error
    // reject('Error occurred');
  }, 1000);
});

Example: Using Promises

fetchData
  .then(data => console.log('Data:', data))
  .catch(error => console.error('Error:', error));

Promise.all():

• Promise.all() is used to execute multiple promises concurrently and wait for all of them to resolve.
• It returns a single promise that resolves when all input promises have resolved, or rejects if any of the input promises reject.

Example: Promise.all()

const promise1 = fetchData();
const promise2 = fetchData();

Promise.all([promise1, promise2])
  .then(([data1, data2]) => console.log('Data:', data1, data2))
  .catch(error => console.error('Error:', error));

 

Key Points

• Promises simplify asynchronous code management and error handling in JavaScript.
• They provide a cleaner alternative to callback-based asynchronous programming.
• Promises are widely used in modern JavaScript development for handling network requests, file I/O, and other asynchronous operations.