CODEWITHSUNDEEP
javascriptcallback
SuperCoolDude1337
SuperCoolDude1337

Reputation: 95

Javascript Callback functions vs just calling functions

So i don't really understand the point of "callback".

Here is an example of callback:

function sayBye(){
   alert("Bye!");
}

function saySeeYou(){
   alert("See you!");
}

function sayHello(name,myfunc){
   alert("Hello");
   myfunc;
}

sayHello("Max",saySeeYou());

Whats the point of passing in a function when you can just call the function? like this code does the exact same:

function sayBye(){
   alert("Bye!");
}

function saySeeYou(){
   alert("See you!");
}

function sayHello(name){
   alert("Hello");
   saySeeYou();
}

sayHello("Max");

Upvotes: 1

Views: 264

Answers (3)

hugomg
hugomg

Reputation: 69934

Whats the point of passing in a function when you can just call the function?

Usually, callbacks Javascript are used in Javascript for code that you want to run in the future. The simplest example is setTimeout: if you call the callback now then the code runs immedieately instead of after 500 ms.

//prints with a delay
console.log("Hello");
setTimeout(function(){
   console.log("Bye");
}, 500);

//no delay this time
console.log("Hello");
console.log("Bye");

Of course, it would be really neat if we could write something along the lines of

//fake Javascript:
console.log("Hello");
wait(500);
console.log("Bye");

But sadly Javascript doesnt let you do that. Javascript is strictly single-threaded so the only way to code the wait function would be to pause the execution of any scripts in the page for 500 ms, which would "freeze" things in an unresponsive state. Because of this, operations that take a long time to complete, like timeouts or AJAX requests usually use callbacks to signal when they are done instead of blocking execution and then returning when done.

By the way, when passing callbacks you should only pass the function name. If you add the parenthesis you are instead calling the function and passing its return value instead:

//When you write
foo(10, mycallback());

//You are actually doing
var res = mycallback();
foo(10, res);
//which will run things in the wrong order

Upvotes: 3

Mritunjay
Mritunjay

Reputation: 25882

You are doing it wrong, you should do like bellow

Don't call the function just pass the function as callback

use this

sayHello("Max",saySeeYou); //here the second parameter is function

instead of

sayHello("Max",saySeeYou());//This will put the result of saySeeYou as second parameter

in say hello call the functiom

function sayHello(name,myfunc){
    console.log("Hello");
    myfunc();
}

Upvotes: 0

Claudio Redi
Claudio Redi

Reputation: 68400

Your code is not correct as Felix Kling already pointed out. Besides this, passing a function instead of calling one directly allows you to insert different behavior, your code is more decoupled and flexible. Here an example:

function sayBye(){    
    alert("Bye!");    
}

function saySeeYou(){    
    alert("See you!");   
}

function sayHello(name,myfunc){    
    alert("Hello");
    if (myfunc) {  
       myfunc();    
    }
}

sayHello("Max",saySeeYou);

// I'm inserting a different behavior. Now instead of displayng "See you!" 
// will show "Bye!". 
sayHello("Max",sayBye);

Upvotes: 0

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