You're not using this properly. In JavaScript, this references the parent object of the method at call-time. So for example:
function exampleMethod() {
console.log(this.name);
}
let obj1 = {"name": "obj1"};
let obj2 = {"name": "obj2"};
obj1.example = exampleMethod;
obj2.example = exampleMethod;
obj1.example(); // "obj1" because the "parent" to this call is obj1
obj2.example(); // "obj2" because the "parent" to this call is obj2
let example = obj1.example;
example(); // undefined because there is no "parent" to example() in this call
So when you do getInventoryContents(foo, bar, this.createOffer), when the module receives the callback, it sees this:
TradeOfferManager.prototype.getInventoryContents(blah, callback) {
// do stuff
callback(); // no parent! so "this" will not be what you expect
};
What you should do instead is: getInventoryContents(foo, bar, this.createOffer.bind(this)). Using the bind method "binds" the value of this before call-time.
