QUESTION: Что такое callbacks?

A callback, or callback function, is a traditional approach to organizing asynchronous programming in JavaScript. The core concept is that a regular function is passed as an argument to another function and is called by the latter after a specific operation completes or a given event occurs.

Classic examples of using callbacks include file reading operations, timers, or sending network requests. For example, when requesting data from a server, you pass a function that will only trigger when the response arrives. In standard Node.js practice, it is common to use the error-first callbacks approach. In this case, the first argument passed to the callback function is reserved for an error object. If an error occurred during the operation, it will be passed to this argument. If everything went successfully, the first argument will be null or undefined, and the useful data will be passed in the second and subsequent arguments.

However, using pure callbacks has a significant drawback known as callback hell. When you need to perform multiple asynchronous operations sequentially, one after another, developers are forced to deeply nest functions inside each other. Such code becomes extremely difficult to read, test, and maintain, and it greatly complicates error handling.

It is precisely because of these architectural issues that the community has gradually moved away from the widespread use of callbacks in favor of more advanced constructs, such as Promises and async/await. Despite this, understanding how callback functions work remains critically important, as they are still deeply integrated into many older libraries and fundamental browser API methods, such as addEventListener event listeners.