HyperScript
Docs

From Skript

Paste a Skript script and get it back in HyperScript. The common things are rewritten for you: events, command options, arguments, variables, lists, messages and comparisons. What the converter does not know stays as it was, for you to finish.

This will not always work.The converter goes line by line and knows the common things. A simple script usually comes out right. A complex one will not: menus, anything from a Skript addon, event values, nested loops over lists, functions with default values and long conditions come out half converted or wrong. Always read the result and check it in the playground before it goes on a server.
Nothing you paste leaves your browser, except to be checked for mistakes.

What is rewritten

What it does not do

Addon syntax (SkBee, skript-gui, Skellett and the like) is not known to it. The Docs say how HyperScript writes menus, scoreboards and the rest.