Problem in Chapter 04.0, line 351

[stefanos82]

I'm currently reading the PDF (nice work by the way!) and when I tested the code, line 351 worked which baffled me; after a couple of reads, I have realized I didn't have 'use strict'; at the very top of my file, which is not mentioned anywhere in the book.

learn-nodejs-hard-way/chapters/ch04.0-logtar-our-logging-library.md

Lines 340 to 354 in 33b587c

	Let's try to test this.
	```js
	new Logger("OK"); // throws error
	new Logger(LogLevel.Debug); // works fine
	new Logger(); // works fine
	let logger = new Logger(LogLevel.Warn);
	logger.level; // returns the `level` because of the getter `level()`
	logger.#level; // throws error
	logger.#level = LogLevel.Warn; // throws error
	logger.level = 10; // throws error
	```
	Perfect! This all looks really good. We are confident that neither clients nor our own library's code will affect the internals of the library. Please note that only the `#level` member variable can be changed from within the class Logger's scope, which is exactly what we want.