HaxeFlixel coding Language
No, you will have to use Haxe (https://haxe.org/).
As @Gama11 said, to use HaxeFlixel, you'll have to use the Haxe language.
If you want to stick with JS and do game development, then I would recommend Phaser.
Or you could go the other way and make your JS with Haxe.
Best of luck!
@antwgames It does limit them to HTML5. Also, HaxeFlixel has no explicit support for this / no TS type definitions, so this is only really an option with pure OpenFL.