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skills/n8n-expression-syntax/COMMON_MISTAKES.md

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+# Common n8n Expression Mistakes
+
+Complete catalog of expression errors with explanations and fixes.
+
+---
+
+## 1. Missing Curly Braces
+
+**Problem**: Expression not recognized, shows as literal text
+
+❌ **Wrong**:
+```
+$json.email
+```
+
+✅ **Correct**:
+```
+{{$json.email}}
+```
+
+**Why it fails**: n8n treats text without {{ }} as a literal string. Expressions must be wrapped to be evaluated.
+
+**How to identify**: Field shows exact text like "$json.email" instead of actual value.
+
+---
+
+## 2. Webhook Body Access
+
+**Problem**: Undefined values when accessing webhook data
+
+❌ **Wrong**:
+```
+{{$json.name}}
+{{$json.email}}
+{{$json.message}}
+```
+
+✅ **Correct**:
+```
+{{$json.body.name}}
+{{$json.body.email}}
+{{$json.body.message}}
+```
+
+**Why it fails**: Webhook node wraps incoming data under `.body` property. The root `$json` contains headers, params, query, and body.
+
+**Webhook structure**:
+```javascript
+{
+  "headers": {...},
+  "params": {...},
+  "query": {...},
+  "body": {         // User data is HERE!
+    "name": "John",
+    "email": "john@example.com"
+  }
+}
+```
+
+**How to identify**: Webhook workflow shows "undefined" for fields that are definitely being sent.
+
+---
+
+## 3. Spaces in Field Names
+
+**Problem**: Syntax error or undefined value
+
+❌ **Wrong**:
+```
+{{$json.first name}}
+{{$json.user data.email}}
+```
+
+✅ **Correct**:
+```
+{{$json['first name']}}
+{{$json['user data'].email}}
+```
+
+**Why it fails**: Spaces break dot notation. JavaScript interprets space as end of property name.
+
+**How to identify**: Error message about unexpected token, or undefined when field exists.
+
+---
+
+## 4. Spaces in Node Names
+
+**Problem**: Cannot access other node's data
+
+❌ **Wrong**:
+```
+{{$node.HTTP Request.json.data}}
+{{$node.Respond to Webhook.json}}
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}
+{{$node["Respond to Webhook"].json}}
+```
+
+**Why it fails**: Node names are treated as object property names and need quotes when they contain spaces.
+
+**How to identify**: Error like "Cannot read property 'Request' of undefined"
+
+---
+
+## 5. Incorrect Node Reference Case
+
+**Problem**: Undefined or wrong data returned
+
+❌ **Wrong**:
+```
+{{$node["http request"].json.data}}  // lowercase
+{{$node["Http Request"].json.data}}  // wrong capitalization
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}  // exact match
+```
+
+**Why it fails**: Node names are **case-sensitive**. Must match exactly as shown in workflow.
+
+**How to identify**: Undefined value even though node exists and has data.
+
+---
+
+## 6. Double Wrapping
+
+**Problem**: Literal {{ }} appears in output
+
+❌ **Wrong**:
+```
+{{{$json.field}}}
+```
+
+✅ **Correct**:
+```
+{{$json.field}}
+```
+
+**Why it fails**: Only one set of {{ }} is needed. Extra braces are treated as literal characters.
+
+**How to identify**: Output shows "{{value}}" instead of just "value".
+
+---
+
+## 7. Array Access with Dots
+
+**Problem**: Syntax error or undefined
+
+❌ **Wrong**:
+```
+{{$json.items.0.name}}
+{{$json.users.1.email}}
+```
+
+✅ **Correct**:
+```
+{{$json.items[0].name}}
+{{$json.users[1].email}}
+```
+
+**Why it fails**: Array indices require brackets, not dots. Number after dot is invalid JavaScript.
+
+**How to identify**: Syntax error or "Cannot read property '0' of undefined"
+
+---
+
+## 8. Using Expressions in Code Nodes
+
+**Problem**: Literal string instead of value, or errors
+
+❌ **Wrong (in Code node)**:
+```javascript
+const email = '{{$json.email}}';
+const name = '={{$json.body.name}}';
+```
+
+✅ **Correct (in Code node)**:
+```javascript
+const email = $json.email;
+const name = $json.body.name;
+
+// Or using Code node API
+const email = $input.item.json.email;
+const allItems = $input.all();
+```
+
+**Why it fails**: Code nodes have **direct access** to data. The {{ }} syntax is for expression fields in other nodes, not for JavaScript code.
+
+**How to identify**: Literal string "{{$json.email}}" appears in Code node output instead of actual value.
+
+---
+
+## 9. Missing Quotes in $node Reference
+
+**Problem**: Syntax error
+
+❌ **Wrong**:
+```
+{{$node[HTTP Request].json.data}}
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}
+```
+
+**Why it fails**: Node names must be quoted strings inside brackets.
+
+**How to identify**: Syntax error "Unexpected identifier"
+
+---
+
+## 10. Incorrect Property Path
+
+**Problem**: Undefined value
+
+❌ **Wrong**:
+```
+{{$json.data.items.name}}       // items is an array
+{{$json.user.email}}            // user doesn't exist, it's userData
+```
+
+✅ **Correct**:
+```
+{{$json.data.items[0].name}}    // access array element
+{{$json.userData.email}}        // correct property name
+```
+
+**Why it fails**: Wrong path to data. Arrays need index, property names must be exact.
+
+**How to identify**: Check actual data structure using expression editor preview.
+
+---
+
+## 11. Using = Prefix Outside JSON
+
+**Problem**: Literal "=" appears in output
+
+❌ **Wrong (in text field)**:
+```
+Email: ={{$json.email}}
+```
+
+✅ **Correct (in text field)**:
+```
+Email: {{$json.email}}
+```
+
+**Note**: The `=` prefix is **only** needed in JSON mode or when you want to set entire field value to expression result:
+
+```javascript
+// JSON mode (set property to expression)
+{
+  "email": "={{$json.body.email}}"
+}
+
+// Text mode (no = needed)
+Hello {{$json.body.name}}!
+```
+
+**Why it fails**: The `=` is parsed as literal text in non-JSON contexts.
+
+**How to identify**: Output shows "=john@example.com" instead of "john@example.com"
+
+---
+
+## 12. Expressions in Webhook Path
+
+**Problem**: Path doesn't update, validation error
+
+❌ **Wrong**:
+```
+path: "{{$json.user_id}}/webhook"
+path: "users/={{$env.TENANT_ID}}"
+```
+
+✅ **Correct**:
+```
+path: "my-webhook"              // Static paths only
+path: "user-webhook/:userId"    // Use dynamic URL parameters instead
+```
+
+**Why it fails**: Webhook paths must be static. Use dynamic URL parameters (`:paramName`) instead of expressions.
+
+**How to identify**: Webhook path doesn't change or validation warns about invalid path.
+
+---
+
+## 13. Forgetting .json in $node Reference
+
+**Problem**: Undefined or wrong data
+
+❌ **Wrong**:
+```
+{{$node["HTTP Request"].data}}          // Missing .json
+{{$node["Webhook"].body.email}}         // Missing .json
+```
+
+✅ **Correct**:
+```
+{{$node["HTTP Request"].json.data}}
+{{$node["Webhook"].json.body.email}}
+```
+
+**Why it fails**: Node data is always under `.json` property (or `.binary` for binary data).
+
+**How to identify**: Undefined value when you know the node has data.
+
+---
+
+## 14. String Concatenation Confusion
+
+**Problem**: Attempting JavaScript template literals
+
+❌ **Wrong**:
+```
+`Hello ${$json.name}!`          // Template literal syntax
+"Hello " + $json.name + "!"     // String concatenation
+```
+
+✅ **Correct**:
+```
+Hello {{$json.name}}!           // n8n expressions auto-concatenate
+```
+
+**Why it fails**: n8n expressions don't use JavaScript template literal syntax. Adjacent text and expressions are automatically concatenated.
+
+**How to identify**: Literal backticks or + symbols appear in output.
+
+---
+
+## 15. Empty Expression Brackets
+
+**Problem**: Literal {{}} in output
+
+❌ **Wrong**:
+```
+{{}}
+{{ }}
+```
+
+✅ **Correct**:
+```
+{{$json.field}}                 // Include expression content
+```
+
+**Why it fails**: Empty expression brackets have nothing to evaluate.
+
+**How to identify**: Literal "{{ }}" text appears in output.
+
+---
+
+## Quick Reference Table
+
+| Error | Symptom | Fix |
+|-------|---------|-----|
+| No {{ }} | Literal text | Add {{ }} |
+| Webhook data | Undefined | Add `.body` |
+| Space in field | Syntax error | Use `['field name']` |
+| Space in node | Undefined | Use `["Node Name"]` |
+| Wrong case | Undefined | Match exact case |
+| Double {{ }} | Literal braces | Remove extra {{ }} |
+| .0 array | Syntax error | Use [0] |
+| {{ }} in Code | Literal string | Remove {{ }} |
+| No quotes in $node | Syntax error | Add quotes |
+| Wrong path | Undefined | Check data structure |
+| = in text | Literal = | Remove = prefix |
+| Dynamic path | Doesn't work | Use static path |
+| Missing .json | Undefined | Add .json |
+| Template literals | Literal text | Use {{ }} |
+| Empty {{ }} | Literal braces | Add expression |
+
+---
+
+## Debugging Process
+
+When expression doesn't work:
+
+1. **Check braces**: Is it wrapped in {{ }}?
+2. **Check data source**: Is it webhook data? Add `.body`
+3. **Check spaces**: Field or node name has spaces? Use brackets
+4. **Check case**: Does node name match exactly?
+5. **Check path**: Is the property path correct?
+6. **Use expression editor**: Preview shows actual result
+7. **Check context**: Is it a Code node? Remove {{ }}
+
+---
+
+**Related**: See [EXAMPLES.md](EXAMPLES.md) for working examples of correct syntax.