When working with asynchronous JavaScript, you often use callbacks to handle tasks that take time to complete, such as reading files or making network requests. However, when you need to perform several asynchronous operations in sequence—where each depends on the result of the previous one—you can quickly run into a problem known as callback hell. This happens when callbacks are nested within other callbacks, creating code that is deeply indented and hard to follow. As your code grows, this nesting makes it difficult to read, debug, and maintain.

This code shows how quickly callback hell can develop. Each asynchronous function is nested inside the callback of the previous one. This "pyramid" structure not only reduces readability but also makes it easy to introduce bugs. If you need to add error handling or change the order of operations, you may have to rewrite large sections of code. These challenges led to the introduction of alternative patterns like Promises, which help flatten the structure and improve code clarity.