Asynchronous code is a fundamental part of JavaScript, allowing you to perform operations like fetching data from a server, reading files, or waiting for timers without blocking the rest of your program. In JavaScript, asynchronous behavior is commonly handled using callbacks, promises, or the modern async/await syntax. Unlike synchronous code, where you get results right away, asynchronous functions might take some time to complete, so their results are not available immediately. This timing difference means you need special techniques to test asynchronous code properly: your tests must wait for the async operations to finish before making assertions, or you could end up with false positives or negatives.