Luau

Roblox long ran stock Lua 5.1, but as the platform grew the company needed both more performance and tooling for large codebases. The result was its own interpreter and a language that stays backward compatible with 5.1 while adding optional type annotations, a static type checker, string interpolation and extras like continue and compound assignment.

Just as important is the sandbox: Roblox runs code from millions of unknown authors, so the standard library is stripped of anything that reaches the file system or the OS. The source was released under the MIT licence in 2021 and has since been picked up outside Roblox, including as an embedded scripting language in other games — the same role GDScript plays in Godot.