function - Why callback is always passed as last parameter JavaScript - Stack Overflow

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In Javascript, I have seen the callback function is passed as the last parameter I am curious why so? Is it a good practice or standard way?

For example:

var doSomething = function(fname, lname, callback){
  console.log("Your name is :"+ fname +" "+ lname);
  callback();
}

var callback = function(){
  console.log("Your name is printed successfully."):
}

doSomething('Arpit', 'Meena', callback); // callback is last parameter here

I know we can pass it at any position and it works but I just want to know the reason behind this.

Thanks.

In Javascript, I have seen the callback function is passed as the last parameter I am curious why so? Is it a good practice or standard way?

For example:

var doSomething = function(fname, lname, callback){
  console.log("Your name is :"+ fname +" "+ lname);
  callback();
}

var callback = function(){
  console.log("Your name is printed successfully."):
}

doSomething('Arpit', 'Meena', callback); // callback is last parameter here

I know we can pass it at any position and it works but I just want to know the reason behind this.

Thanks.

• javascript
• function
• callback

Share Improve this question Follow edited Jan 29, 2018 at 11:06 Arpit Kumar asked Dec 30, 2016 at 10:28 Arpit KumarArpit Kumar 2,24955 gold badges3131 silver badges5656 bronze badges 3

• What makes you think that it is passed last in most cases. Can you give some example – Panther Commented Dec 30, 2016 at 10:30
• May be its most plex or different type of argument , people tend to add in last – Panther Commented Dec 30, 2016 at 10:31
• In asynchronous function and mostly in Node js we generally use function(req, res, next){} where next is the callback function. – Arpit Kumar Commented Dec 30, 2016 at 10:34

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2 Answers 2

Sorted by: Reset to default 10

The reason I do this way and have seen others doing so is because of code readability, when you have a calback as the last argument, you can declare the callback inline without making the code unreadable. For example, if you pass the callback as the first parameter:

var doSomething = function(callback, fname, lname){
  console.log("Your name is :"+ fname +" "+ lname);
  callback();
}

You have to call it like:

doSomething(function callback(){
  console.log('foo');
}, 'Arpit', 'Meena');

However, if you use it as the last argument, things are kept much more clear on the function call:

var doSomething = function(fname, lname, callback){
  console.log("Your name is :"+ fname +" "+ lname);
  callback();
}

doSomething('Arpit', 'Meena', function callback(){
  console.log('foo');
});

It is standard JavaScript convention. Part of the reason it has bee standard practice is that it makes things more readable. It's like defining properties at the top of a class - you don't have to but it's standard practice.

Suggested reading: http://technosophos./2012/08/22/javascript-callbacks-function-last%E2%80%A6-or-lost.html

http://hancic.info/callback-parameter-should-always-be-last

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