Another basic example:
NOT working:
var img = new Image;
img.onload = function() {
this.myGlobalFunction(img);
};
img.src = reader.result;
Working:
var img = new Image;
img.onload = function() {
this.myGlobalFunction(img);
}.bind(this);
img.src = reader.result;
So basically: just add .bind(this) to your function