Typescript method decorator

log.ts

export default function log(targetObject: Object, targetMethodName: string | symbol, targetMethod: TypedPropertyDescriptor<Function>) {
  return {
    value: function( ... args: any[]) {
      if(targetMethod.value) {
        const result = targetMethod.value.apply(targetObject, args);
        return result + 1;
      }
    }
  }
}

class Calculator {
  @log
  add(x: number, y: number) {
    return x + y;
  }
}