javascript - Why I can't directly set console.log() as callback function -

why code doesn't work

function callback(num, func) {     for(var = 0; < num; i++) {         func();     } }  callback(4, console.log("hello")); 

i know have this:

callback(4, function() { console.log("hello"); }); 

but still don't understand reason why have that.

lets step through visualise going on. when interpreter reads code, evaluates function expressions inside-out. say:

callback(4, console.log("hello")); //          ^------------------^---- evaluated before "callback" called. 

a longer-form way of writing produces same outcome be:

var result = console.log("hello"); callback(4, result); 

the console.log function doesn't have defined return value (or @ least not standardised one) - though incidentally functions in javascript return something - when unspecified value literally undefined. when run code calling:

callback(4, undefined); 

this means inside callback, trying call func function result in:

typeerror: undefined not function

this why need encapsulate logic in new function-closure - whereby func obtains reference callable function object.

---

so how can tidy code?

generally in simple cases this, have done solve problem acceptable more complex logic, can end several levels of nested callbacks (sometimes referred "callback-soup"). improve readability code reuse common can introduce function-factory hides away nastiness, example:

function mylogger(message){     return function(){         console.log(message);     }       }  callback(4, mylogger('hello'));