Do you need to write anything to change JS to ES6?
Is there any code I need to write(such as <!DOCTYPE HTML/>) to switch the language to ES6?
no. you don't need anything. Browsers support ES6 out of the box. Also, better use Babel / Búble / sucrase / swc to compile to ES5 for 100% browser support
Js is suppose to be ES6 or what are you talking about?
no, dont worry.
No, For Unique ES6 Features, There Are Some Codes. You Just Need To Implement Them. But You Don't Have To Do Anything About JS to ES6. Let The Browser Deal With It. Btw Makes Sure Your Browser Is Not Backdated