I know that callback function runs asynchronously, but why?

Question

Which part of syntax provides the information that this function should run in other thread and be non-blocking?

Let\'s consider simple asynchronous I/O in node.js

Answer 1

A callback is not necessarily asynchronous. Execution depends entirely on how fs.readFile decides to treat the function parameter.

In JavaScript, you can execute a function asynchronously using for example setTimeout.

Discussion and resources:

How does node.js implement non-blocking I/O?

Concurrency model and Event Loop

Wikipedia:

There are two types of callbacks, differing in how they control data flow at runtime: blocking callbacks (also known as synchronous callbacks or just callbacks) and deferred callbacks (also known as asynchronous callbacks).