[javascript] Create a custom callback in JavaScript

All I need to do is to execute a callback function when my current function execution ends.

function LoadData() 
{
    alert('The data has been loaded');
    //Call my callback with parameters. For example,
    //callback(loadedData , currentObject);
}

A consumer for this function should be like this:

object.LoadData(success);

function success(loadedData , currentObject) 
{
  //Todo: some action here 
}

How do I implement this?

It is good practice to make sure the callback is an actual function before attempting to execute it:

if (callback && typeof(callback) === "function") {

  callback();
}

My 2 cent. Same but different...

<script>
    dosomething("blaha", function(){
        alert("Yay just like jQuery callbacks!");
    });


    function dosomething(damsg, callback){
        alert(damsg);
        if(typeof callback == "function") 
        callback();
    }
</script>

function loadData(callback) {

    //execute other requirement

    if(callback && typeof callback == "function"){
        callback();
   }
}

loadData(function(){

   //execute callback

});

function LoadData(callback) 
{
    alert('the data have been loaded');
    callback(loadedData, currentObject);
}

Some of the answers, while correct may be a little tricky to understand. Here is an example in layman's terms:

var users = ["Sam", "Ellie", "Bernie"];

function addUser(username, callback)
{
    setTimeout(function()
    {
        users.push(username);
        callback();
    }, 200);
}

function getUsers()
{
    setTimeout(function()
    {
        console.log(users);
    }, 100);
}

addUser("Jake", getUsers);

The callback means, "Jake" is always added to the users before displaying the list of users with console.log.

Source (YouTube)

If you want to execute a function when something is done. One of a good solution is to listen to events. For example, I'll implement a Dispatcher, a DispatcherEvent class with ES6,then:

let Notification = new Dispatcher()
Notification.on('Load data success', loadSuccessCallback)

const loadSuccessCallback = (data) =>{
   ...
}
//trigger a event whenever you got data by
Notification.dispatch('Load data success')

Dispatcher:

class Dispatcher{
  constructor(){
    this.events = {}
  }

  dispatch(eventName, data){
    const event = this.events[eventName]
    if(event){
      event.fire(data)
    }
  }

  //start listen event
  on(eventName, callback){
    let event = this.events[eventName]
    if(!event){
      event = new DispatcherEvent(eventName)
      this.events[eventName] = event
    }
    event.registerCallback(callback)
  }

  //stop listen event
  off(eventName, callback){
    const event = this.events[eventName]
    if(event){
      delete this.events[eventName]
    }
  }
}

DispatcherEvent:

class DispatcherEvent{
  constructor(eventName){
    this.eventName = eventName
    this.callbacks = []
  }

  registerCallback(callback){
    this.callbacks.push(callback)
  }

  fire(data){
    this.callbacks.forEach((callback=>{
      callback(data)
    }))
  }
}

Happy coding!

p/s: My code is missing handle some error exceptions

When calling the callback function, we could use it like below:

consumingFunction(callbackFunctionName)

Example:

// Callback function only know the action,
// but don't know what's the data.
function callbackFunction(unknown) {
  console.log(unknown);
}

// This is a consuming function.
function getInfo(thenCallback) {
  // When we define the function we only know the data but not
  // the action. The action will be deferred until excecuting.
  var info = 'I know now';
  if (typeof thenCallback === 'function') {
    thenCallback(info);    
  }
}

// Start.
getInfo(callbackFunction); // I know now

This is the Codepend with full example.

   function callback(e){
      return e;
   }
    var MyClass = {
       method: function(args, callback){
          console.log(args);
          if(typeof callback == "function")
          callback();
       }    
    }

==============================================

MyClass.method("hello",function(){
    console.log("world !");
});

==============================================

Result is:

hello world !

Try:

function LoadData (callback)
{
    // ... Process whatever data
    callback (loadedData, currentObject);
}

Functions are first class in JavaScript; you can just pass them around.