How to properly check if a variable is undefined in JavaScript?

Hi everyone! I just started learning JavaScript and ran into a problem. I need to check whether a variable is defined or not to avoid errors in my code. I tried different methods, but they behave differently, and I'm confused. Here's an example of my code: ```javascript let myVar; console.log(myVar); // undefined if (myVar == undefined) { console.log('Variable is not defined'); } if (typeof myVar === 'undefined') { console.log('Also not defined'); } ``` But when I try to check a non-existent variable (one I haven't even declared), I get a ReferenceError. For example: ```javascript if (someVar === undefined) { console.log('Won't work'); } ``` Also, when I use `==` and `===` — is there a difference? I've also heard about `void` and that you can compare with