I know that writing async functions is recommended in nodejs. However, I feel it's not so nescessary to write some non IO events asynchronously. My code can get less convenient. For example:
//sync
function now(){
return new Date().getTime();
}
console.log(now());
//async
function now(callback){
callback(new Date().getTime());
}
now(function(time){
console.log(time);
});
Does sync method block CPU in this case? Is this remarkable enough that I should use async instead?
Async style is necessary if the method being called can block for a long time waiting for IO. As the node.js event loop is single-threaded you want to yield to the event loop during an IO. If you didn't do this there could be only one IO outstanding at each point in time. That would lead to total non-scalability.
Using callbacks for CPU work accomplishes nothing. It does not unblock the event loop. In fact, for CPU work it is not possible to unblock the event loop. The CPU must be occupied for a certain amount of time and that is unavoidable. (Disregarding things like web workers here).
Callbacks are nothing good. You use them when you have to. They are a necessary consequence of the node.js event loop IO model.
That said, if you later plan on introducing IO into now you might eagerly use a callback style even if not strictly necessary. Changing from synchronous calls to callback-based calls later can be time-consuming because the callback style is viral.
By adding a callback to a function's signature, the code communicates that something asynchronous might happen in this function and the function will call the callback with an error and/or result object.
In case a function does nothing asynchronous and does not involve conditions where a (non programming) error may occur don't use a callback function signature but simply return the computation result.
Functions with callbacks are not very convenient to handle by the caller so avoid callbacks until you really need them.