Usage of Javascript callback function

Question

I think this is so basic so people maybe minus votes on this document, but even so this is so confused me about callback function in JavaScript.

function doSomething(callback){ 
 setTimeout(hello,5000);
 callback();
}
function hi(){
 console.log("hi");
}
function hello(){
 console.log("hello");
}
doSomething(hi);
/* result */
// hi
// (after 5 seconds) hello

I want to use callback function as a handle function's execute order, so I decided use callback pattern. In above code, I think after 5 seconds, the callback function should be executed, but why callback ignore before function and was ran first? Could you tell me a some hint.

Thanks.

• Sterling Archer: Because your timeout doesn't include the callback, it runs on its own

Answer 1

In your code callback() was executing after the execution of the line setTimeout() but the callback of setTimeout will trigger after 5000ms, that is the expected behaviour. So if you want callback() to exeute after hello() do:

function doSomething(callback){ 
 setTimeout(function(){
 hello();
 callback();
 },5000);
}