Agent skill

n8n-expression-syntax

Validate n8n expression syntax and fix common errors. Use when writing n8n expressions, using {{}} syntax, accessing $json/$node variables, troubleshooting expression errors, or working with webhook data in workflows.

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SKILL.md

n8n Expression Syntax

Expert guide for writing correct n8n expressions in workflows.

Expression Format

All dynamic content in n8n uses double curly braces:

{{expression}}

Examples:

✅ {{$json.email}}
✅ {{$json.body.name}}
✅ {{$node["HTTP Request"].json.data}}
❌ $json.email  (no braces - treated as literal text)
❌ {$json.email}  (single braces - invalid)

Core Variables

$json - Current Node Output

Access data from the current node:

javascript
{{$json.fieldName}}
{{$json['field with spaces']}}
{{$json.nested.property}}
{{$json.items[0].name}}

$node - Reference Other Nodes

Access data from any previous node:

javascript
{{$node["Node Name"].json.fieldName}}
{{$node["HTTP Request"].json.data}}
{{$node["Webhook"].json.body.email}}

Important:

  • Node names must be in quotes
  • Node names are case-sensitive
  • Must match exact node name from workflow

$now - Current Timestamp

Access current date/time:

javascript
{{$now}}
{{$now.toFormat('yyyy-MM-dd')}}
{{$now.toFormat('HH:mm:ss')}}
{{$now.plus({days: 7})}}

$env - Environment Variables

Access environment variables:

javascript
{{$env.API_KEY}}
{{$env.DATABASE_URL}}

🚨 CRITICAL: Webhook Data Structure

Most Common Mistake: Webhook data is NOT at the root!

Webhook Node Output Structure

javascript
{
  "headers": {...},
  "params": {...},
  "query": {...},
  "body": {           // ⚠️ USER DATA IS HERE!
    "name": "John",
    "email": "john@example.com",
    "message": "Hello"
  }
}

Correct Webhook Data Access

javascript
❌ WRONG: {{$json.name}}
❌ WRONG: {{$json.email}}

✅ CORRECT: {{$json.body.name}}
✅ CORRECT: {{$json.body.email}}
✅ CORRECT: {{$json.body.message}}

Why: Webhook node wraps incoming data under .body property to preserve headers, params, and query parameters.

Common Patterns

Access Nested Fields

javascript
// Simple nesting
{{$json.user.email}}

// Array access
{{$json.data[0].name}}
{{$json.items[0].id}}

// Bracket notation for spaces
{{$json['field name']}}
{{$json['user data']['first name']}}

Reference Other Nodes

javascript
// Node without spaces
{{$node["Set"].json.value}}

// Node with spaces (common!)
{{$node["HTTP Request"].json.data}}
{{$node["Respond to Webhook"].json.message}}

// Webhook node
{{$node["Webhook"].json.body.email}}

Combine Variables

javascript
// Concatenation (automatic)
Hello {{$json.body.name}}!

// In URLs
https://api.example.com/users/{{$json.body.user_id}}

// In object properties
{
  "name": "={{$json.body.name}}",
  "email": "={{$json.body.email}}"
}

When NOT to Use Expressions

❌ Code Nodes

Code nodes use direct JavaScript access, NOT expressions!

javascript
// ❌ WRONG in Code node
const email = '={{$json.email}}';
const name = '{{$json.body.name}}';

// ✅ CORRECT in Code node
const email = $json.email;
const name = $json.body.name;

// Or using Code node API
const email = $input.item.json.email;
const allItems = $input.all();

❌ Webhook Paths

javascript
// ❌ WRONG
path: "{{$json.user_id}}/webhook"

// ✅ CORRECT
path: "user-webhook"  // Static paths only

❌ Credential Fields

javascript
// ❌ WRONG
apiKey: "={{$env.API_KEY}}"

// ✅ CORRECT
Use n8n credential system, not expressions

Validation Rules

1. Always Use {{}}

Expressions must be wrapped in double curly braces.

javascript
❌ $json.field
✅ {{$json.field}}

2. Use Quotes for Spaces

Field or node names with spaces require bracket notation:

javascript
❌ {{$json.field name}}
✅ {{$json['field name']}}

❌ {{$node.HTTP Request.json}}
✅ {{$node["HTTP Request"].json}}

3. Match Exact Node Names

Node references are case-sensitive:

javascript
❌ {{$node["http request"].json}}  // lowercase
❌ {{$node["Http Request"].json}}  // wrong case
✅ {{$node["HTTP Request"].json}}  // exact match

4. No Nested {{}}

Don't double-wrap expressions:

javascript
❌ {{{$json.field}}}
✅ {{$json.field}}

Common Mistakes

For complete error catalog with fixes, see COMMON_MISTAKES.md

Quick Fixes

Mistake Fix
$json.field {{$json.field}}
{{$json.field name}} {{$json['field name']}}
{{$node.HTTP Request}} {{$node["HTTP Request"]}}
{{{$json.field}}} {{$json.field}}
{{$json.name}} (webhook) {{$json.body.name}}
'={{$json.email}}' (Code node) $json.email

Working Examples

For real workflow examples, see EXAMPLES.md

Example 1: Webhook to Slack

Webhook receives:

json
{
  "body": {
    "name": "John Doe",
    "email": "john@example.com",
    "message": "Hello!"
  }
}

In Slack node text field:

New form submission!

Name: {{$json.body.name}}
Email: {{$json.body.email}}
Message: {{$json.body.message}}

Example 2: HTTP Request to Email

HTTP Request returns:

json
{
  "data": {
    "items": [
      {"name": "Product 1", "price": 29.99}
    ]
  }
}

In Email node (reference HTTP Request):

Product: {{$node["HTTP Request"].json.data.items[0].name}}
Price: ${{$node["HTTP Request"].json.data.items[0].price}}

Example 3: Format Timestamp

javascript
// Current date
{{$now.toFormat('yyyy-MM-dd')}}
// Result: 2025-10-20

// Time
{{$now.toFormat('HH:mm:ss')}}
// Result: 14:30:45

// Full datetime
{{$now.toFormat('yyyy-MM-dd HH:mm')}}
// Result: 2025-10-20 14:30

Data Type Handling

Arrays

javascript
// First item
{{$json.users[0].email}}

// Array length
{{$json.users.length}}

// Last item
{{$json.users[$json.users.length - 1].name}}

Objects

javascript
// Dot notation (no spaces)
{{$json.user.email}}

// Bracket notation (with spaces or dynamic)
{{$json['user data'].email}}

Strings

javascript
// Concatenation (automatic)
Hello {{$json.name}}!

// String methods
{{$json.email.toLowerCase()}}
{{$json.name.toUpperCase()}}

Numbers

javascript
// Direct use
{{$json.price}}

// Math operations
{{$json.price * 1.1}}  // Add 10%
{{$json.quantity + 5}}

Advanced Patterns

Conditional Content

javascript
// Ternary operator
{{$json.status === 'active' ? 'Active User' : 'Inactive User'}}

// Default values
{{$json.email || 'no-email@example.com'}}

Date Manipulation

javascript
// Add days
{{$now.plus({days: 7}).toFormat('yyyy-MM-dd')}}

// Subtract hours
{{$now.minus({hours: 24}).toISO()}}

// Set specific date
{{DateTime.fromISO('2025-12-25').toFormat('MMMM dd, yyyy')}}

String Manipulation

javascript
// Substring
{{$json.email.substring(0, 5)}}

// Replace
{{$json.message.replace('old', 'new')}}

// Split and join
{{$json.tags.split(',').join(', ')}}

Debugging Expressions

Test in Expression Editor

  1. Click field with expression
  2. Open expression editor (click "fx" icon)
  3. See live preview of result
  4. Check for errors highlighted in red

Common Error Messages

"Cannot read property 'X' of undefined" → Parent object doesn't exist → Check your data path

"X is not a function" → Trying to call method on non-function → Check variable type

Expression shows as literal text → Missing {{ }} → Add curly braces

Expression Helpers

Available Methods

String:

  • .toLowerCase(), .toUpperCase()
  • .trim(), .replace(), .substring()
  • .split(), .includes()

Array:

  • .length, .map(), .filter()
  • .find(), .join(), .slice()

DateTime (Luxon):

  • .toFormat(), .toISO(), .toLocal()
  • .plus(), .minus(), .set()

Number:

  • .toFixed(), .toString()
  • Math operations: +, -, *, /, %

Best Practices

✅ Do

  • Always use {{ }} for dynamic content
  • Use bracket notation for field names with spaces
  • Reference webhook data from .body
  • Use $node for data from other nodes
  • Test expressions in expression editor

❌ Don't

  • Don't use expressions in Code nodes
  • Don't forget quotes around node names with spaces
  • Don't double-wrap with extra {{ }}
  • Don't assume webhook data is at root (it's under .body!)
  • Don't use expressions in webhook paths or credentials

Related Skills

  • n8n MCP Tools Expert: Learn how to validate expressions using MCP tools
  • n8n Workflow Patterns: See expressions in real workflow examples
  • n8n Node Configuration: Understand when expressions are needed

Summary

Essential Rules:

  1. Wrap expressions in {{ }}
  2. Webhook data is under .body
  3. No {{ }} in Code nodes
  4. Quote node names with spaces
  5. Node names are case-sensitive

Most Common Mistakes:

  • Missing {{ }} → Add braces
  • {{$json.name}} in webhooks → Use {{$json.body.name}}
  • {{$json.email}} in Code → Use $json.email
  • {{$node.HTTP Request}} → Use {{$node["HTTP Request"]}}

For more details, see:

  • COMMON_MISTAKES.md - Complete error catalog
  • EXAMPLES.md - Real workflow examples

Need Help? Reference the n8n expression documentation or use n8n-mcp validation tools to check your expressions.

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