Synchronous vs Asynchronous

Synchronous (or sync) execution usually refers to code executing in sequence. In sync programming, the program is executed line by line, one line at a time. Each time a function is called, the program execution waits until that function returns before continuing to the next line of code.

Asynchronous (or async) execution refers to execution that doesn’t run in the sequence it appears in the code. In async programming, the program doesn’t wait for the task to complete and can move on to the next task.

In the following example, the sync operation causes the alerts to fire in sequence. In the async operation, while alert(2) appears to execute second, it doesn’t.

An async operation is often I/O related, although setTimeout is an example of something that isn’t I/O but still async. Generally speaking, anything computation-related is sync and, anything input/output/timing-related is async. The reason for I/O operations to be done asynchronously is that they are very slow and would block further execution of code otherwise.