Callbacks

A callback function is a function that is passed as an argument to another function, with the intention of being 'called back' (executed) at a later time. This is the foundational pattern for handling asynchronous operations in JavaScript. In an async scenario, you initiate an operation (e.g., a data request) and provide a callback function. Your code continues to execute without waiting. When the operation eventually completes, the host environment (like the browser or Node.js) places the callback function into the message queue. The event loop then picks it up and executes it. This pattern allows JavaScript to remain non-blocking. The most common convention for callbacks, especially in Node.js, is the 'error-first' pattern, where the first argument to the callback is reserved for an error object. If the operation fails, this argument is populated; otherwise, it's `null`, and the subsequent arguments contain the successful result. While callbacks are fundamental, they have a major drawback. When dealing with multiple dependent asynchronous operations, you end up nesting callbacks inside each other, leading to a pyramid-shaped, deeply indented structure known as 'callback hell' or the 'pyramid of doom'. This code is difficult to read, reason about, and maintain, which is why modern JavaScript has moved towards Promises and async/await.