javascript - Accessing variables in the parent class -

i've been stumped on problem few hours now, , can't seem find solution. have class extends parent class, cannot seem access variables declared in parent's constructor.

i borrowed inheritance technique. uses "extend" function create subclass:

//in functions.js function extend(base, sub, methods) {     sub.prototype = object.create(sub.prototype);     sub.prototype.constructor = sub;     sub.base = base.prototype;      for(var name in methods) { sub.prototype[name] = methods[name]; }      return sub; } 

i create class called stimulus serves function:

//in classes.js function stimulus(module_id, unit_id, attributes) {     this.attributes = attributes;     this.module_id = module_id;     this.unit_id = unit_id;     //create other class variables based on this.attributes, this.module_id, , this.unit_id }  stimulus.prototype = {     _getstimulus: function() { //retrieve database }     //other functions here } 

and have subclass. technique use create borrowed above link:

//in classes.js imagestimulus = (function() {     var $this = function(module_id, unit_id, attributes) {         $this.base.constructor.call(this, module_id, unit_id, attributes);     };      extend(stimulus, $this, {         initialize: function() {             this.fixation_cross = this.attributes['fixation cross'] ? this.attributes['fixation cross'] : false;             //do other stuff         }         //other functions here     });      return $this; })(); 

it seems straightforward enough. however, in main script, when try run this, create object , try run initialize() function , falls apart:

//in main.js var stimulus_objects = [];  for(var = 0; < somelimit; i++) {     //module_id passed directly function     var unit_id = //some source;     var stimulus_attributes = //some source;      stimulus_objects[i] = new imagestimulus(module_id, unit_id, stimulus_attributes);     stimulus_objects[i].initialize(); } 

if check console see says

uncaught typeerror: cannot read property 'fixation cross' of undefined

and corresponds line in imagestimulus.initialize() try call on this.attributes['fixation cross'].

it seems going wrong in making stimulus prototype of imagestimulus, because imagestimulus.initialize() cannot access this.attributes variables created in constructor stimulus class.

does else see error?

i have decent amount of oop programming in java, c++, , php, first attempt @ javascript oop, , feel i'm making simple mistake.

edit: solved problem... somehow.

so seems there trivial solution. stimulus function never being called, , should have been called on line $this.base.constructor.call(). in stimulus.prototype object added constructor: stimulus , stimulus being called properly. seems odd had (shouldn't stimulus() it's own constructor?), works!

stimulus.prototype = {      constructor: stimulus,      _getstimulus: function() {... 

does know why occurred , why fix worked? i'm trying understand did.

i it. you're replacing entire prototype, killing constructor. setting constructor explicitly, putting back. alternative set prototype method directly rather setting entire prototype.

for fiddle useful, bring console , set break point before clicking run. http://jsfiddle.net/x2v7wv6j/

//in functions.js function extend(base, sub, methods) {     sub.prototype = object.create(sub.prototype);     sub.prototype.constructor = sub;     sub.base = base.prototype;      for(var name in methods) { sub.prototype[name] = methods[name]; }      return sub; }  //in classes.js function stimulus(module_id, unit_id, attributes) {     this.attributes = attributes;     this.module_id = module_id;     this.unit_id = unit_id;     //create other class variables based on this.attributes, this.module_id, , this.unit_id }  stimulus.prototype._getstimulus = function() { //retrieve database  } //other functions here  //in classes.js imagestimulus = (function() {     var $this = function(module_id, unit_id, attributes) {         $this.base.constructor.call(this, module_id, unit_id, attributes);     };      extend(stimulus, $this, {         initialize: function() {             this.fixation_cross = this.attributes['fixation cross'] ? this.attributes['fixation cross'] : false;             //do other stuff         }         //other functions here     });      return $this; })();   var foo = function (module_id) {     var stimulus_objects = [];      var somelimit = 10;      for(var = 0; < somelimit; i++) {         //module_id passed directly function         var unit_id = "some source";//some source;         var stimulus_attributes = "some source"; //some source;          stimulus_objects[i] = new imagestimulus(module_id, unit_id, stimulus_attributes);         stimulus_objects[i].initialize();     } }  foo(1);