purpose of callbacks in javascript

Question

Maybe someone mark my question as duplicate or else but i am confusing about callback in JavaScript. I read from here a following piece of code

getText = function(url, callback) // How can I use this callback?
{
var request = new XMLHttpRequest();
request.onreadystatechange = function()
{
    if (request.readyState == 4 && request.status == 200)
    {
        callback(request.responseText); // Another callback here
    }
}; 
request.open('GET', url);
request.send();
}
function mycallback(data) {
    alert(data);
}
getText('somephpfile.php', mycallback); //passing mycallback as a method

and now if i change the above code and remove callbacks like following

getText = function(url) //Also remove from here  How can I use this callback?
{
var request = new XMLHttpRequest();
request.onreadystatechange = function()
{
    if (request.readyState == 4 && request.status == 200)
    {
        mycallback(request.responseText); //Now here i simply call that function
    }
}; 
request.open('GET', url);
request.send();
}
function mycallback(data) {
    alert(data);
}
getText('somephpfile.php'); //Remove (passing mycallback as a method)

So now what is difference now? If no difference then why use callbacks

possible duplicate of Why use callback in JavaScript, what are its advantages? — Ash
– Ash, Commented Mar 4, 2014 at 13:56

Bergi · Accepted Answer · 2014-03-04 14:06:49Z

1

So now what is difference now? If no difference then why use callbacks

Your first function is more general (reusable). You can do all of these:

getText('somephpfile.php', console.log); // or
getText('somephpfile.php', alert); // or
getText('somephpfile.php', mycallback); // or
getText('somephpfile.php', yourcallback); // or
getText('somephpfile.php', whatever);

which will behave differently. In your second function, the getText cannot be used for anything else but alerting ajax data.

answered Mar 4, 2014 at 14:06

Bergi

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3 Comments

Bergi Over a year ago

What exactly is it that you're unclear about? Not having a parameter for the callback - which is effectively removing the "callback" - is like not having a return statement in your function.

Blu Over a year ago

I send a function as a parameter to call later as a callback what if I call actual function where I call callback and not pass as a parameter in a function

Bergi Over a year ago

That's like putting more code in a normal function, but still having it not return results to its caller.

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