Introduction to Asynchronous Programming

Some operations finish in a fraction of a millisecond, such as adding two numbers or reading a property of an object. Others take far longer, such as waiting for a server to answer, reading a large file, or waiting for a timer. A program spends most of that time doing nothing except waiting.

JavaScript runs your code on a single thread, which means it executes one statement at a time and never two at once. If that one thread stopped and waited for every slow operation, nothing else could happen in the meantime