string-safe-inference.ts

declare global {
  interface String {
    toUpperCase<T extends string>(this: T): Uppercase<T>;
    toLowerCase<T extends string>(this: T): Lowercase<T>;
  }
}

const x = 'foo';
const y = x.toUpperCase();
const z = y.toLowerCase();

if(x === z) {
  console.log('x and z are equal');
}

console.log({ x, y, z });