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Zhongwei Li
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skills/n8n-expression-syntax/SKILL.md
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skills/n8n-expression-syntax/SKILL.md
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---
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name: n8n-expression-syntax
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description: 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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---
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# n8n Expression Syntax
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Expert guide for writing correct n8n expressions in workflows.
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---
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## Expression Format
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All dynamic content in n8n uses **double curly braces**:
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```
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{{expression}}
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```
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**Examples**:
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```
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✅ {{$json.email}}
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✅ {{$json.body.name}}
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✅ {{$node["HTTP Request"].json.data}}
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❌ $json.email (no braces - treated as literal text)
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❌ {$json.email} (single braces - invalid)
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```
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---
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## Core Variables
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### $json - Current Node Output
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Access data from the current node:
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```javascript
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{{$json.fieldName}}
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{{$json['field with spaces']}}
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{{$json.nested.property}}
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{{$json.items[0].name}}
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```
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### $node - Reference Other Nodes
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Access data from any previous node:
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```javascript
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{{$node["Node Name"].json.fieldName}}
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{{$node["HTTP Request"].json.data}}
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{{$node["Webhook"].json.body.email}}
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```
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**Important**:
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- Node names **must** be in quotes
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- Node names are **case-sensitive**
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- Must match exact node name from workflow
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### $now - Current Timestamp
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Access current date/time:
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```javascript
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{{$now}}
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{{$now.toFormat('yyyy-MM-dd')}}
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{{$now.toFormat('HH:mm:ss')}}
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{{$now.plus({days: 7})}}
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```
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### $env - Environment Variables
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Access environment variables:
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```javascript
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{{$env.API_KEY}}
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{{$env.DATABASE_URL}}
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```
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---
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## 🚨 CRITICAL: Webhook Data Structure
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**Most Common Mistake**: Webhook data is **NOT** at the root!
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### Webhook Node Output Structure
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```javascript
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{
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"headers": {...},
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"params": {...},
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"query": {...},
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"body": { // ⚠️ USER DATA IS HERE!
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"name": "John",
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"email": "john@example.com",
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"message": "Hello"
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}
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}
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```
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### Correct Webhook Data Access
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```javascript
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❌ WRONG: {{$json.name}}
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❌ WRONG: {{$json.email}}
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✅ CORRECT: {{$json.body.name}}
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✅ CORRECT: {{$json.body.email}}
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✅ CORRECT: {{$json.body.message}}
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```
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**Why**: Webhook node wraps incoming data under `.body` property to preserve headers, params, and query parameters.
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---
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## Common Patterns
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### Access Nested Fields
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```javascript
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// Simple nesting
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{{$json.user.email}}
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// Array access
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{{$json.data[0].name}}
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{{$json.items[0].id}}
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// Bracket notation for spaces
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{{$json['field name']}}
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{{$json['user data']['first name']}}
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```
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### Reference Other Nodes
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```javascript
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// Node without spaces
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{{$node["Set"].json.value}}
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// Node with spaces (common!)
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{{$node["HTTP Request"].json.data}}
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{{$node["Respond to Webhook"].json.message}}
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// Webhook node
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{{$node["Webhook"].json.body.email}}
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```
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### Combine Variables
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```javascript
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// Concatenation (automatic)
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Hello {{$json.body.name}}!
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// In URLs
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https://api.example.com/users/{{$json.body.user_id}}
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// In object properties
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{
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"name": "={{$json.body.name}}",
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"email": "={{$json.body.email}}"
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}
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```
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---
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## When NOT to Use Expressions
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### ❌ Code Nodes
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Code nodes use **direct JavaScript access**, NOT expressions!
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```javascript
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// ❌ WRONG in Code node
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const email = '={{$json.email}}';
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const name = '{{$json.body.name}}';
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// ✅ CORRECT in Code node
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const email = $json.email;
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const name = $json.body.name;
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// Or using Code node API
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const email = $input.item.json.email;
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const allItems = $input.all();
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```
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### ❌ Webhook Paths
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```javascript
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// ❌ WRONG
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path: "{{$json.user_id}}/webhook"
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// ✅ CORRECT
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path: "user-webhook" // Static paths only
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```
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### ❌ Credential Fields
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```javascript
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// ❌ WRONG
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apiKey: "={{$env.API_KEY}}"
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// ✅ CORRECT
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Use n8n credential system, not expressions
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```
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---
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## Validation Rules
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### 1. Always Use {{}}
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Expressions **must** be wrapped in double curly braces.
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```javascript
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❌ $json.field
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✅ {{$json.field}}
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```
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### 2. Use Quotes for Spaces
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Field or node names with spaces require **bracket notation**:
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```javascript
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❌ {{$json.field name}}
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✅ {{$json['field name']}}
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❌ {{$node.HTTP Request.json}}
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✅ {{$node["HTTP Request"].json}}
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```
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### 3. Match Exact Node Names
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Node references are **case-sensitive**:
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```javascript
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❌ {{$node["http request"].json}} // lowercase
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❌ {{$node["Http Request"].json}} // wrong case
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✅ {{$node["HTTP Request"].json}} // exact match
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```
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### 4. No Nested {{}}
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Don't double-wrap expressions:
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```javascript
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❌ {{{$json.field}}}
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✅ {{$json.field}}
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```
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---
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## Common Mistakes
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For complete error catalog with fixes, see [COMMON_MISTAKES.md](COMMON_MISTAKES.md)
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### Quick Fixes
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| Mistake | Fix |
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|---------|-----|
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| `$json.field` | `{{$json.field}}` |
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| `{{$json.field name}}` | `{{$json['field name']}}` |
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| `{{$node.HTTP Request}}` | `{{$node["HTTP Request"]}}` |
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| `{{{$json.field}}}` | `{{$json.field}}` |
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| `{{$json.name}}` (webhook) | `{{$json.body.name}}` |
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| `'={{$json.email}}'` (Code node) | `$json.email` |
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---
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## Working Examples
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For real workflow examples, see [EXAMPLES.md](EXAMPLES.md)
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### Example 1: Webhook to Slack
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**Webhook receives**:
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```json
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{
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"body": {
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"name": "John Doe",
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"email": "john@example.com",
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"message": "Hello!"
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}
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}
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```
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**In Slack node text field**:
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```
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New form submission!
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Name: {{$json.body.name}}
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Email: {{$json.body.email}}
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Message: {{$json.body.message}}
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```
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### Example 2: HTTP Request to Email
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**HTTP Request returns**:
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```json
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{
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"data": {
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"items": [
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{"name": "Product 1", "price": 29.99}
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]
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}
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}
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```
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**In Email node** (reference HTTP Request):
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```
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Product: {{$node["HTTP Request"].json.data.items[0].name}}
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Price: ${{$node["HTTP Request"].json.data.items[0].price}}
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```
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### Example 3: Format Timestamp
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```javascript
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// Current date
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{{$now.toFormat('yyyy-MM-dd')}}
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// Result: 2025-10-20
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// Time
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{{$now.toFormat('HH:mm:ss')}}
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// Result: 14:30:45
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// Full datetime
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{{$now.toFormat('yyyy-MM-dd HH:mm')}}
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// Result: 2025-10-20 14:30
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```
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---
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## Data Type Handling
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### Arrays
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```javascript
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// First item
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{{$json.users[0].email}}
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// Array length
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{{$json.users.length}}
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// Last item
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{{$json.users[$json.users.length - 1].name}}
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```
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### Objects
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```javascript
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// Dot notation (no spaces)
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{{$json.user.email}}
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// Bracket notation (with spaces or dynamic)
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{{$json['user data'].email}}
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```
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### Strings
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```javascript
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// Concatenation (automatic)
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Hello {{$json.name}}!
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// String methods
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{{$json.email.toLowerCase()}}
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{{$json.name.toUpperCase()}}
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```
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### Numbers
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```javascript
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// Direct use
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{{$json.price}}
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// Math operations
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{{$json.price * 1.1}} // Add 10%
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{{$json.quantity + 5}}
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```
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---
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## Advanced Patterns
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### Conditional Content
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```javascript
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// Ternary operator
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{{$json.status === 'active' ? 'Active User' : 'Inactive User'}}
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// Default values
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{{$json.email || 'no-email@example.com'}}
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```
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### Date Manipulation
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```javascript
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// Add days
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{{$now.plus({days: 7}).toFormat('yyyy-MM-dd')}}
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// Subtract hours
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{{$now.minus({hours: 24}).toISO()}}
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// Set specific date
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{{DateTime.fromISO('2025-12-25').toFormat('MMMM dd, yyyy')}}
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```
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### String Manipulation
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```javascript
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// Substring
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{{$json.email.substring(0, 5)}}
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// Replace
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{{$json.message.replace('old', 'new')}}
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// Split and join
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{{$json.tags.split(',').join(', ')}}
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```
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---
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## Debugging Expressions
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### Test in Expression Editor
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1. Click field with expression
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2. Open expression editor (click "fx" icon)
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3. See live preview of result
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4. Check for errors highlighted in red
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### Common Error Messages
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**"Cannot read property 'X' of undefined"**
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→ Parent object doesn't exist
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→ Check your data path
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**"X is not a function"**
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→ Trying to call method on non-function
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→ Check variable type
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**Expression shows as literal text**
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→ Missing {{ }}
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→ Add curly braces
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---
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## Expression Helpers
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### Available Methods
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**String**:
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- `.toLowerCase()`, `.toUpperCase()`
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- `.trim()`, `.replace()`, `.substring()`
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- `.split()`, `.includes()`
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**Array**:
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- `.length`, `.map()`, `.filter()`
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- `.find()`, `.join()`, `.slice()`
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**DateTime** (Luxon):
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- `.toFormat()`, `.toISO()`, `.toLocal()`
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- `.plus()`, `.minus()`, `.set()`
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**Number**:
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- `.toFixed()`, `.toString()`
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- Math operations: `+`, `-`, `*`, `/`, `%`
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---
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## Best Practices
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### ✅ Do
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- Always use {{ }} for dynamic content
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- Use bracket notation for field names with spaces
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- Reference webhook data from `.body`
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- Use $node for data from other nodes
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- Test expressions in expression editor
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### ❌ Don't
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- Don't use expressions in Code nodes
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- Don't forget quotes around node names with spaces
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- Don't double-wrap with extra {{ }}
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- Don't assume webhook data is at root (it's under .body!)
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- Don't use expressions in webhook paths or credentials
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---
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## Related Skills
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- **n8n MCP Tools Expert**: Learn how to validate expressions using MCP tools
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- **n8n Workflow Patterns**: See expressions in real workflow examples
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- **n8n Node Configuration**: Understand when expressions are needed
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---
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## Summary
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**Essential Rules**:
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1. Wrap expressions in {{ }}
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2. Webhook data is under `.body`
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3. No {{ }} in Code nodes
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4. Quote node names with spaces
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5. Node names are case-sensitive
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**Most Common Mistakes**:
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- Missing {{ }} → Add braces
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- `{{$json.name}}` in webhooks → Use `{{$json.body.name}}`
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- `{{$json.email}}` in Code → Use `$json.email`
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- `{{$node.HTTP Request}}` → Use `{{$node["HTTP Request"]}}`
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For more details, see:
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- [COMMON_MISTAKES.md](COMMON_MISTAKES.md) - Complete error catalog
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- [EXAMPLES.md](EXAMPLES.md) - Real workflow examples
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---
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**Need Help?** Reference the n8n expression documentation or use n8n-mcp validation tools to check your expressions.
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Reference in New Issue
Block a user