Promises in javaScript

In JavaScript, a promise is an object that represents the eventual completion or failure of an asynchronous operation and its resulting value. It's a way to handle asynchronous operations more easily and avoid the complexities of callback-based approaches.

A promise can be in one of three states:

  1. Pending: Initial state, representing that the operation hasn’t completed yet.
  2. Fulfilled: The operation completed successfully, and the promise now has a resolved value.
  3. Rejected: The operation failed, and the promise has a reason for the failure.

Here's a basic example of creating and using a promise:

            // Creating a promise
            let myPromise = new Promise((resolve, reject) => {
              // Asynchronous operation (e.g., fetching data)
              let operationCompletedSuccessfully = true; // Change to false to see rejection
              if (operationCompletedSuccessfully) {
                // If successful, resolve with a value
                resolve("Operation completed successfully!");
              } else {
                // If failed, reject with a reason
                reject("Operation failed!");
            // Using the promise
              .then((result) => {
                // If the promise is resolved
                console.log("Success:", result);
              .catch((error) => {
                // If the promise is rejected
                console.log("Error:", error);

Here's a breakdown:

  • A promise is created with the new Promise() constructor, which takes a callback function with two parameters: resolve and reject. Inside this callback, you perform the asynchronous operation.
  • If the operation succeeds, you call resolve() and pass the result. If it fails, you call reject() and pass an error.
  • To handle the result of the promise, you use the .then() method. If the promise is fulfilled, the success handler (passed to .then()) will be executed.
  • To handle errors or rejections, you use the .catch() method. This catches any errors that occur during the promise execution.

Promises provide a cleaner way to work with asynchronous code compared to nested callbacks. They also support chaining multiple asynchronous operations sequentially using .then() and handling errors using .catch(), making the code more readable and maintainable.

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