I am trying to set a variable declared at the beginning of the class (a boolean) to true once a callback is called, but I keep getting a TypeScript erorr.

Here is the error:

TypeError: Cannot set property 'nonReceived' of undefined

Here is my code:

  finalizeToken(){

  braintree.setup(JSON.parse(this.finalToken), 'dropin', {

     container: 'dropin-container',

     defaultFirst: true,

      form: 'checkout-form',

      onPaymentMethodReceived: function (obj) {

        this.nonReceived = true;

      localStorage.setItem('nonce', obj.nonce);

    }

  });  

}

The brintree-setup connect to Braintree Payments, and awaits user payment info. Once they submit the form, I need the variable this.nonReceived to be set to true.