SUPER:
function baseClass(config){
  this.config = config;
  this.setConfig = function(value){};
}

MIXIN:
function observableMixin(){
  this.listeners = {};
  this.addListener = function(name, callback){
    if(!this.listeners[name]) this.listeners[name] = [];
    this.listeners[name].push(callback);
  }
  this.removeListener = function(name, callback){
    this.listeners[name] = this.listeners[name].filter(
      function(fn){ 
        return fn != callback;
        });
  }
}

TypeError: c.hasListener is not a function
    at it
    at begin
    at it
    at describe
    at /runner/frameworks/javascript/cw-2.js:152:11
    at Promise._execute
    at Promise._resolveFromExecutor
    at new Promise
    at describe
    at /home/codewarrior/index.js:49:5
    at /home/codewarrior/index.js:96:5
    at Object.handleError

There's no hasListener method anywhere, and no explanation of what the tests are trying to do - how am I even supposed to debug this?

Edit: turns out there's a observableMixin.prototype.hasListener method, but again, if your solution is wrong and doesn't add this method to the created class, you will likely not be able to tell what's wrong.

I'd recommend adding some example code in the description. Show how you expect the extend function to be invoked, with or without mixin and scope.

Also, the function's argument names in the description doesn't match up with the argument names in the solution setup:

extend('ChildClass', BaseClass, EventDispatcher, scope);
function extend(className, superClass, mixin, scope)

You should change one or both so that the argument names are consistent. The latter names are clearer IMHO.

Then, change your bullet list so that it has a bullet item for each argument, describing the type (string, function, object), what it represents, and identifying the arguments which were optional.