Synchronous and Asynchronous callback functions in ES6 and Node

Question

I'm new to ES6 and Node.js

I would like to understand how callback functions work in a synchronous environment and asynchronous environment.

I tried to run the following in Node:

function func1() { 
    console.log('hello'); 
}

func1(function(){ 
    console.log('world');
});

From my understanding of asynchronous callbacks, func1 should execute first, and when func1 finishes execution it calls back the function inside its argument list.

So in this case:

console.log('world')

should execute in second place, but i'm only getting as an output:hello.

Am i missing something? Can someone explain to me with clear examples how callbacks work? Is it the same in a synchronous environment?

`function func1(callback) { console.log('hello'); callback(); }` — 4castle, Oct 28 '17 at 01:16
The occurrence of a callback does not imply asynchronous code. Your example is not asynchronous. — Felix Kling, Oct 30 '17 at 00:06

Mark · Answer 1 · 2017-10-28T02:04:32.467

You should start by examining your func1() function:

function func1() { 
    console.log('hello'); 
}

The most important thing to recognize is that it takes zero arguments. The result is that it doesn't matter what you pass to it. For example:

func1("world")
func1(1)
func1(false)
func1(function (){console.log('world)}

all do exactly the same thing because func1() ignores everything passed in as an argument.

Functions that don't ignore their parameters are what you need if you want to pass a callback, because they need to do something with that callback. So a function taking a callback should have a signature more like this:

function func1(some_argument){
   // do something with argument
}

If you're expecting that argument to be a function, the natural things to do would be to execute it like:

function func1(some_function){
    // call that function
    some_function()
}

So for your example, you would end up with:

function func1(callback) { 
    console.log('hello');
    callback() 
}

func1(function(){ 
    console.log('world');
});

which would result in two console.log()s.

Also, since it was in the title, there isn't anything asynchronous going on here.

score 1 · Accepted Answer · answered Oct 28 '17 at 01:23

So when you run func1(function(){...}) you're actually passing the unnamed or 'anonymous' function as an argument to func1. To function as a callback however, you need to explicitly execute it.

function func1(fn) { 
    console.log('hello'); 
    fn();
}

func1(function(){ 
    console.log('world');
});

In other node libraries, the developers are most likely handling the callbacks you give to their functions, which is why they work.

Hope this helps!

Synchronous and Asynchronous callback functions in ES6 and Node

2 Answers2