Answer by Gayrat Vlasov for Typescript method is return undefined with method decorator

This is a full working example:

function log(msg: string): MethodDecorator {  return (    target: Object,    key: string | symbol,    descriptor: PropertyDescriptor  ) => {    let originalMethod = descriptor.value;    descriptor.value = function (...args: any[]) {      //before      if (args.length >0)  console.log(`${msg} ${key.toString()} method called with args: ${JSON.stringify(args)}`);      let result = originalMethod!.apply(this, args);      //after      if (result) console.log(`${key.toString()} method return value: ${JSON.stringify(result)}`);    };    return descriptor;  };}class Book {  constructor(    public id: number,    public title: string,    public publisher: string  ) {}  @log('setting ')  setId(id: number) {    this.id = id;  }  @log('getting ')  getId(): number {    return this.id;  }}const a = new Book(1, "title", "publisher");a.setId(2);a.getId();

You can run it at typescript playground