Promises In JavaScript

A promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation. It acts as a placeholder for a future value and provides a way to handle the result when it becomes available.

Concepts:

Asynchronous Operation: A task that takes some time to complete without blocking the main thread of execution.

Pending: Initial state of a promise, indicating the operation is ongoing.

Resolved: The operation completed successfully, and the promise holds the resulting value.

Rejected: The operation failed, and the promise holds the error reason.

Settled: A promise that is either resolved or rejected (not pending).

Using Promises:

1.Create a Promise:

Use the Promise constructor with an executor function.

The executor function defines the asynchronous operation.

It takes two arguments: resolve and reject callbacks.

Call resolve with the result when the operation finishes successfully.

Call reject with an error if the operation fails.

2.Handle the Promise:

Use the .then method to handle the resolved value.

The .then method takes a callback function that receives the resolved value.

Use the .catch method to handle any errors during the operation.

The .catch method takes a callback function that receives the error reason.

Benefits of Promises:

Improved code readability and maintainability: Separates asynchronous logic from the main flow.

Error handling: Provides a structured way to handle errors in asynchronous operations.

Chaining Promises:Allows chaining multiple asynchronous operations together.

Example:

function fetchData(url) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      const data = { message: "Data from " + url };
      resolve(data); // Simulate successful data retrieval after a delay
    }, 2000);
  });
}

fetchData("https://example.com/api/data")
  .then(data => {
    console.log("Data received:", data);
  })
  .catch(error => {
    console.error("Error fetching data:", error);
  });

console.log("This line executes immediately");

In this example:

fetchData returns a promise that simulates fetching data from a URL.

We call fetchData and use .then to log the resolved data.

.catch handles any errors that might occur during data retrieval.