JSFiddle - React, Tailwind, and code Playground

JavaScript

var A = function(arg){
    this.storedArg = arg;
    var that = this;
    this.callback = function(){ alert(that.storedArg); }
}

var B = function() {
    this.doCallback = function(callback){ callback(); }
}       

var pubCallback = function(){ alert('Public callback') };

var a = new A(123);
var b = new B();

b.doCallback(pubCallback); // works as expected
b.doCallback(a.callback);  // want 123, get undefined