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COMMON_MISTAKES.md
COMMON_MISTAKES.md
 
 
EXAMPLES.md
EXAMPLES.md
 
 
README.md
README.md
 
 
SKILL.md
SKILL.md
 
 

README.md

n8n Expression Syntax

Expert guide for writing correct n8n expressions in workflows.

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

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