JSFiddle - React, Tailwind, and code Playground

JavaScript

var MyClass = function() {
    this.customer = null;
};

// Any functions need to be added to the prototype,
// and should use the keyword this to access member fields.
MyClass.prototype.test = function(data){
    alert('testing');
};

// At this point, MyClass is a constructor function with all of it's
// prototype methods set, ready to be instantiated.

var testClass = new MyClass();
testClass.customer = 'John B'; // May also want to consider moving this into the constructor function as a parameter.
testClass.test();