How to handle asynchronous operations with promises

26 Oct 2023

asynchronous

Asynchronous operations are a common requirement in modern software development. When dealing with asynchronous code, it is crucial to handle the sequencing and error handling properly to ensure a smooth execution flow. One approach for handling asynchronous operations is by using promises.

What are promises?

Promises are a JavaScript feature introduced in ECMAScript 6 (ES6) that provide a way to manage asynchronous operations. They represent the eventual completion or failure of an asynchronous task and allow you to handle the result or error once it becomes available.

A promise can be in one of three states:

Pending: The initial state. The promise is neither fulfilled nor rejected.
Fulfilled: The asynchronous operation has completed successfully, and the promise has a resolved value.
Rejected: The asynchronous operation has failed, and the promise has a reason for rejection.

Using promises for asynchronous operations

To handle asynchronous operations using promises, you need to create a promise and chain it with .then() and .catch() methods.

Creating a promise

You can create a promise using the Promise constructor. The constructor takes a single argument, a callback function, which provides two parameters: resolve and reject. Inside this callback function, you can perform your asynchronous operation and call resolve with the result or reject with an error if something goes wrong.

Here’s an example of creating a promise that resolves after a delay of 1 second:

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

Handling promise resolution

Once you have a promise, you can handle its resolution using the .then() method. The .then() method takes a callback function as an argument, which will be called when the promise is fulfilled. The resolved value will be passed as an argument to this callback function.

myPromise.then((result) => {
  console.log(result); // Output: "Operation completed"
});

Handling promise rejection

If the promise is rejected, you can handle the error using the .catch() method. The .catch() method takes a callback function that will be called when the promise is rejected. The reason for rejection will be passed as an argument to this callback function.

myPromise.catch((error) => {
  console.error(error); // Output: "Something went wrong!"
});

Chaining promises

Promises can be chained together to handle multiple asynchronous operations sequentially. Each .then() block can return a new promise, allowing you to control the flow of execution.

myPromise
  .then((result) => {
    console.log(result); // Output: "Operation completed"
    return anotherAsyncOperation();
  })
  .then((result) => {
    console.log(result); // Output: "Another operation completed"
  })
  .catch((error) => {
    console.error(error); // Output: "Something went wrong!"
  });

Conclusion

Promises provide an elegant way to handle asynchronous operations in JavaScript. By using promises, you can ensure that your code executes in the desired order and handle errors effectively. They have become an essential tool for managing asynchronous tasks in modern web development.

For more information on promises, you can refer to the MDN Web Docs or the JavaScript Promises - Explained video. #javascript #asynchronous