Are all Node.js callback functions asynchronous?

Question

I'm working on learning Node.js and all I hear in every tutorial is "Node is asynchronous and no -blocking!"

I've heard in regular browser JavaScript only certain things such as AJAX calls can be made asynchronous or non-blocking (using callbacks)... Is this true of Node.js as well, or are all Node.js callback functions made asynchronous/non-blocking?

Answer 1

are all Node.js callback functions made asynchronous/non-blocking?

No. Only I/O is usually asynchronous, but many other callbacks are synchronous. Always check the docs.

Examples of async functions:

• Async Filesystem access (they have sync counterparts without callbacks, though)
• Timers (setTimeout)
• process.nextTick, setImmediate
• most database connections
• network connections
• Promises

Examples of sync callbacks:

• EventEmitter (depends on when the event is fired)
• Array iteration methods like forEach
• Array sort comparator callbacks
• String replace match callbacks

See also Are all javascript callbacks asynchronous? If not, how do I know which are? (including some other examples).