Are JS/TS limited to html5 or do they support all platforms?

Do JS/TS support all platforms like Haxe does?

They’re limited to HTML5. See also OpenFL’s readme:

The NPM version of OpenFL is designed to be used in a browser environment.

You would use a wrapper such as Electron, PhoneGap or Cordova to support multiple platforms. It would still run in a browser environment, but could still be used on multiple platforms