Comprehensive guide for Google Apps Script development covering all built-in services (SpreadsheetApp, DocumentApp, GmailApp, DriveApp, CalendarApp, FormApp, SlidesApp), triggers, authorisation, error handling, and performance optimisation. Use when automating Google Sheets operations, creating Google Docs, managing Gmail/email, working with Google Drive files, automating Calendar events, implementing triggers (time-based, event-based), building custom functions, creating add-ons, handling OAuth scopes, optimising Apps Script performance, working with UrlFetchApp for API calls, using PropertiesService for persistent storage, or implementing CacheService for temporary data. Covers batch operations, error recovery, and JavaScript ES6+ runtime. Do NOT use for standalone Node.js scripts, Google Cloud Functions, Cloud Run, or any non-Apps-Script JavaScript runtime - those use different APIs and quotas.