JSFiddle - React, Tailwind, and code Playground

JavaScript

// ctor for foo object
function foo(callback) {
    
    // do slow initialization here..
    
    // callback when done
    callback();
};

foo.prototype = function() {
    
    return {
        // doStuff method
        doStuff: function() {
          alert('stuff done');
        }
    };        
}();

// instantiate the foo object, passing in the callback
var f = new foo(function() {
    
    //Uncaught TypeError: Cannot call method 'doStuff' of undefined 
    f.doStuff();
    
});