n8n-expression-syntax

n8n Expression Syntax

Expert guide for writing correct n8n expressions in workflows.

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Purpose

Teaches correct n8n expression syntax ({{ }} patterns) and fixes common mistakes, especially the critical webhook data structure gotcha.

Activates On

• expression
• {{}} syntax
• $json, $node, $now, $env
• webhook data
• troubleshoot expression error
• undefined in workflow

File Count

4 files

Dependencies

n8n-mcp tools:

• None directly (syntax knowledge skill)
• Works with n8n-mcp validation tools

Related skills:

• n8n Workflow Patterns (uses expressions in examples)
• n8n MCP Tools Expert (validates expressions)
• n8n Node Configuration (when expressions are needed)

Coverage

Core Topics

• Expression format ({{ }})
• Core variables ($json, $node, $now, $env)
• Webhook data structure ($json.body.*)
• When NOT to use expressions (Code nodes)
• $jmespath(object, query) — querying nested JSON in one expression, with the quoting rules that fail silently
• Runtime errors inside {{ }} resolve to null (node still succeeds) — how to surface them

Common Patterns

• Accessing nested fields
• Referencing other nodes
• Array and object access
• Date/time formatting
• String manipulation

Error Prevention

• 17 common mistakes with fixes (incl. JMESPath quoting and the silent-null runtime error)
• Quick reference table
• Debugging process

Evaluations

6 scenarios (100% coverage expected):

1. eval-001: Missing curly braces
2. eval-002: Webhook body data access (critical!)
3. eval-003: Code node vs expression confusion
4. eval-004: Node reference syntax
5. eval-005: $jmespath quoting (double-quoted string, bare number)
6. eval-006: Filter drops everything on a green run (silent null)

Key Features

✅ Critical Gotcha Highlighted: Webhook data under .body ✅ Real Examples: From MCP testing and real templates ✅ Quick Fixes Table: Fast reference for common errors ✅ Code vs Expression: Clear distinction ✅ Comprehensive: Covers 95% of expression use cases

Files

• SKILL.md - Main content with all essential knowledge
• COMMON_MISTAKES.md - Complete error catalog with 15 common mistakes
• EXAMPLES.md - 10 real working examples
• README.md (this file) - Skill metadata

Success Metrics

Expected outcomes:

• Users correctly wrap expressions in {{ }}
• Zero webhook .body access errors
• No expressions used in Code nodes
• Correct $node reference syntax

Last Updated

2025-10-20

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Part of: n8n-skills repository Conceived by: Romuald Członkowski - aiadvisors.pl/en