Description: Rejection in the context of JavaScript refers to the state of a promise that has been rejected, indicating that the asynchronous operation has failed. In JavaScript, promises are objects that represent the eventual completion or failure of an asynchronous operation and its resulting value. When a promise is rejected, it means that an error occurred during the execution of the operation, and a reason or cause for this failure is provided. This state is crucial for error handling in programming, as it allows developers to manage unexpected situations more effectively. By using the `.catch()` method of a promise, programmers can capture the error and make appropriate decisions, such as displaying a message to the user or attempting an alternative operation. The rejection of a promise is a fundamental part of the asynchronous programming model, allowing developers to write cleaner and more manageable code, avoiding the so-called ‘callback hell’.
History: The concept of promises in JavaScript was introduced in 2011 with the ECMAScript 5.1 specification, although its use became popular with the arrival of ECMAScript 6 in 2015, which standardized the syntax and behavior of promises. Before this, developers often used callbacks to handle asynchronous operations, which could lead to code that was difficult to read and maintain. The introduction of promises allowed for a more structured way to handle asynchronicity, making it easier to write cleaner code and avoiding common issues like ‘callback hell’.
Uses: Promises are primarily used to handle asynchronous operations in JavaScript, such as HTTP requests, file reading, and other tasks that may take time to complete. By using promises, developers can chain multiple asynchronous operations in a more readable and manageable way. Additionally, promises are fundamental for working with APIs, which allow for simple and efficient network requests.
Examples: A practical example of promise rejection is as follows: when making an HTTP request to a server that is unavailable, the promise is rejected, and the error can be handled using the `.catch()` method. For example: fetch(‘https://api.example.com/data’) .then(response => response.json()) .catch(error => console.error(‘Error:’, error)); In this case, if the request fails, the error is captured and displayed in the console.
