JSFiddle - React, Tailwind, and code Playground

JavaScript

const transform = function (string) {
  if (string === undefined) {
    throw new Error('must provide string');
  }
  const toUpperCase = function () {
    string = string.toUpperCase();
    return this;
  };
  const toLowerCase = function () {
    string = string.toLowerCase();
    return this;
  };
  const capitalizeFirstLetter = function () {
    string = string.charAt(0).toUpperCase() + string.slice(1);
    return this;
  };
  
  const end = function(){
  	return string;
  }

  return {
    toUpperCase,
    toLowerCase,
    capitalizeFirstLetter,
    end,
  };
};

var a = transform('someThinG');
a = a.toLowerCase();
a = a.capitalizeFirstLetter();
a = a.end()

console.log(a)
console.log(typeof a)