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):

eval-001: Missing curly braces
eval-002: Webhook body data access (critical!)
eval-003: Code node vs expression confusion
eval-004: Node reference syntax
eval-005: $jmespath quoting (double-quoted string, bare number)
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

/n8n-expression-syntax

README.md