What is a key difference between synchronous and asynchronous JavaScript?

Synchronous JavaScript executes tasks sequentially, meaning that each task must finish before the next one begins. This can lead to blocking behavior, where a long-running task can delay the execution of subsequent tasks. For example, if a synchronous function is downloading data from a server, the entire program will pause until that data retrieval is complete. This model is straightforward but can lead to inefficiencies, particularly when dealing with operations that might take time, such as network requests or timer functions.

In contrast, asynchronous JavaScript allows certain operations to run independently of the main program flow, enabling the program to continue executing other tasks while waiting for the completion of tasks like data retrieval from a server. This is achieved through mechanisms like callbacks, promises, or async/await patterns. This method improves performance and user experience by preventing the program from freezing during long operations.

The other options do not accurately describe the characteristics of synchronous and asynchronous JavaScript. For instance, asynchronous doesn't necessarily run tasks in order, nor does it wait for a page to load; it simply allows certain tasks to proceed independently. Similarly, synchronous operations are generally blocking by nature, which negates the notion of non-blocking capabilities that are characteristic of asynchronous programming.