Developer Intro to Custom Extensions

Custom extensions come in two forms: Lua extensions and native extensions. Native extensions are more powerful as they have direct access to the SE! source code, and are best used for complex blocks or those with strict performance requirements. Lua extensions are better for simple blocks, as they are easy to program.

Lua Extensions

Lua extensions are broken into two parts: the Lua code itself and a metadata JSON file. As an example, here's the meta.json for the JSON extension:

{
  "$schema": "https://raw.githubusercontent.com/ScratchEverywhere/seec/refs/heads/main/schema.json",
  "core": false,
  "id": "skyhigh173JSON",
  "name": "JSON",
  "description": "Handle JSON strings and arrarys.",
  "permissions": []
}

The format of this file can be found in the schema file.

Compiling a Lua extension is done with seec.

Native Extensions

Native extensions require that you are able to compile SE! itself. In order for a native extension to work on Scratch Everywhere!, it must be built on the same system, using the same compiler and the same arguments, with the same version of Scratch Everywhere!.