How to declare a type that allows null or undefined in TypeScript?

Hi! I just started learning TypeScript and ran into a problem. I have a function that should accept a number, but sometimes I want to pass null or just not pass anything (undefined). In regular JavaScript this works fine, but TypeScript complains. Here's an example of my code: ```typescript function greetUser(age: number) { console.log(`Age: ${age}`); } // Want to pass null if age is unknown greetUser(null); // Error: Argument of type 'null' is not assignable to parameter of type 'number'. // Or not pass the argument at all greetUser(); // Error: Expected 1 arguments, but got 0. ``` I tried using `age: number | null`, but then the function doesn't allow passing undefined. And if I write `age: number | undefined`, it doesn't work with null.