function format(formatString: string) { return function (target: any, propertyKey: string) { let value: string; const getter = () => value; const setter = (newVal: string) => { value = formatString.replace('{}', newVal); }; Object.defineProperty(target, propertyKey, { get: getter, set: setter, enumerable: true, configurable: true }); }; } class Greeter { @format('Hello, {}!') greeting: string; } const greeter = new Greeter(); greeter.greeting = 'World'; console.log(greeter.greeting);