Definite Assignment Assertion operator (!):

If we do not want to initialize members inside a constructor, we use this.

class OKGreeter {
// Not initialized, but no error
name!: string;
}

Index signature:

The idea of index signatures is to type objects of unknown structure when you only know the key and value types. The key of the index signature can only be a string, number, or symbol

For more information: https://dmitripavlutin.com/typescript-index-signatures/-

Order of execution:

The order of class initialization, as defined by JavaScript, is:

• The base class fields are initialized
• The base class constructor runs
• The derived class fields are initialized
• The derived class constructor runs

Subclassing some built-in classes may not work as expected:

Returned object of super() will replace ‘this’ of MsgError. As a result, this.sayHello() will not work.

The solution is to use Object.setPrototypeOf(this, MsgError.prototype) inside constructor after the super() call.

 class MsgError extends Error {
	constructor(m: string) {
		super(m);
	}
	sayHello() {
		return "hello " + this.message;
	}
}

class MySafe {
	private secretKey = 12345;
}
const s = new MySafe();
// Not allowed during type checking
console.log(s.secretKey);
//Property 'secretKey' is private and only accessible within class 'MySafe'.

// OK
console.log(s["secretKey"]);

Handling of ‘this’ is indeed unusual in JS: