Set a callback function inside a javascript class

Question

excuse the pseudo code, my actual file is much larger:/

I want to call a function (with parameters) from inside a class. However, that function should be passed to the class as a variable.

someObject = {
    itWorked:function(answer){
       alert(answer);
    },

    plugins:{
        somePlugin:function(){

            var callback;
            this.doSomething = doSomething;

            function setCallback(c){
                callback = c;
            }

            function doSomething(){
                 var answer = "hello";
                 [callback](answer); // how do I call this?
            }

        }
    },

    widgets:{
        something:function(){
            var doIt = new someObject();
            doIt.setCallback(someObject.itWorked()); // how do I send this?
            doIt.doSomething();
        }
    }
}

So how would I pass itWorked() to the class? And how would I call that itWorked(answer) function within the class as well as passing a variable to if?

Zoidberg · Accepted Answer · 2011-11-27 15:09:47Z

3

You will need to change

setCallback = function (c) {callback = c;}

to

this.setCallback =  function (c) {callback = c;}

so the setCallback function will be public.

If you also want to scope the callback, you can call it like this

callback.call(scope, param1, param2);

If you don't know how many parameters, you can call it like this

callback.apply(scope, parameters);

Scope could be any object, even an empty one {} if you want.

By the way, I really like your use of private variables in this example, great work with the javascript. Here is a good way to write your javascript object to help with the initialization and readability

var mynamespace = {};

(function () {
   function MyObject(param1, param2) {
      this.initialize(param1, param2);
   }

   MyObject.prototype = {
      initialize: function (param1, param2) {
          var privateScope = {
              param1: param1,
              param2: param2,
              callback: null
          };

          this.setCallback = function (c) {
              privateScope.callback = c;
          }

          this.doSomething = function () {
              if (privateScope.callback) {
                  privateScope.callback.call();
              }
          }
      }
   }
   mynamespace.MyObject = MyObject;
}());

Then to use it

var obj = new mynamespace.MyObject("value1", "value2");

edited Nov 27, 2011 at 15:09

answered Nov 27, 2011 at 14:45

Zoidberg

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7 Comments

ed209 Over a year ago

thanks :) I'm a bit of a js noob at this level, thanks for your feedback!

Zoidberg Over a year ago

Well you are off to a great start, this is well written javascript.

ed209 Over a year ago

do you know if there is a __construct() equivalent for javascript? So rather than call doIt.setCallback(someObject.itWorked) pass it in at the new someObject() stage?

Esailija Over a year ago

@ed209, the function someObject is the constructor function. Functions act as constructors when used with new

Didier Ghys Over a year ago

Check this doc about .apply() and .call()

|

JJJ · Accepted Answer · 2011-11-27 14:41:29Z

1

Remove the parentheses to pass the function as a variable.

doIt.setCallback( someObject.itWorked );

You can then use the callback as you would any other function.

callback( answer );

answered Nov 27, 2011 at 14:41

JJJ

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2 Comments

Zoidberg Over a year ago

Good answer here, I just posted an alternative implementation, but there is nothing wrong with this one.

Didier Ghys Over a year ago

You should also check if the callback exists, and is an executable function: if (callback && typeof(callback) === "function") {}

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