n8n JavaScript Code Reference — Data Access, Built-ins, and 10 Production Patterns

Deep reference for writing JavaScript in n8n Code nodes: All Items vs Each Item mode, $input.all() / .first() / .item data access, $helpers.httpRequest, DateTime (Luxon), $jmespath, and persistent storage. Includes 10 production-tested patterns — aggregation, regex filtering, markdown parsing, JSON comparison, CRM transformation, Slack formatting, top-N filtering. Covers the top 5 Code-node errors.