n8n Expression Syntax Guide — {{ }} Patterns, $json, and Webhook Gotchas

Expert reference for n8n expression syntax — {{ }} format, the $json / $node / $now / $env variables, and the critical webhook gotcha (data lives under $json.body, not $json directly). Includes 15 documented common mistakes with before/after fixes, and clarifies when NOT to use {{ }} inside Code nodes. Eliminates 80% of 'undefined' and 'expression error' issues.