I have a js script that works fine on its own, however when I try to move my code to reactjs the nested/referenced function loses 'this' and it references window instead. Pseudo code bellow. Any idea what causes this? I figure its related to webpack or something reactjs is doing?

lib.AnMovieClip = function() {
 this.testVariable = true;
 this.gotoAndPlay = function(){
  // 'this' works here perfectly fine
  this.otherFunction();
 }
 this.otherFunction(){
  // returns undefined, as 'this' is now the window object
  console.log(this.testVariable); 
 }
}