The built-in code editor - line numbers, JavaScript syntax highlighting, tab-to-indent. Opens validate.js/process.js from Project Files, but works as a plain text editor for any file, including layout JSON you'd rather hand-edit directly.

• Save enables the moment there's something unsaved.
• Closing with unsaved changes prompts to discard or go back and save.

For what actually goes in validate.js/process.js, see Scripting & Processing.