n8n Expression Error Diagnosis

Diagnose and fix expression evaluation failures in n8n v1.x workflows. For error type details see references/methods.md. For before/after fixes see references/examples.md. For anti-patterns see references/anti-patterns.md.

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Quick Diagnostic Table

Symptom	Cause	Fix
undefined when accessing $json.field	Field does not exist on current item	Check field name spelling; use $json?.field or $ifEmpty($json.field, fallback)
TypeError: Cannot read properties of undefined	Accessing nested field on null/undefined parent	Chain optional access: $json.parent?.child?.value
$itemIndex is not defined in Code node	$itemIndex is NOT available in Code node	Use items.indexOf(item) or loop index variable instead
$secrets is not defined in Code node	$secrets is NOT available in Code node	Use $env.SECRET_NAME or pass secret via preceding Set node
$response is not defined	$response used outside HTTP Request node	ONLY use $response in HTTP Request node parameter fields
Paired item not found	Item linking broken between nodes	Use $("<Node>").first() or $("<Node>").all()[index] instead of .item
JMESPath returns unexpected result	Parameter order swapped	ALWAYS use $jmespath(object, searchString) — object first, search second
Expression returns empty string	Field exists but value is null, "", or undefined	Use $ifEmpty($json.field, "default") to provide fallback
$getWorkflowStaticData returns empty	Static data not available during manual test	ALWAYS test static data with active workflow (webhook/trigger), NEVER manual execution
TypeError: X is not a function	Calling n8n extension method on wrong type	Verify data type: .extractEmail() requires string, .average() requires array
$("<Node>").item returns wrong data	Expression evaluated in non-matching context	Use $("<Node>").itemMatching(index) in Code node; use .first() or .all() for explicit access
Python AttributeError on item access	Using dot notation in Python Code node	ALWAYS use bracket notation: item["json"]["field"]
$pageCount is not defined	Used outside HTTP Request node pagination	$pageCount is ONLY available in HTTP Request node
Number treated as string in comparison	n8n expression returned string type	Explicitly convert: Number($json.price) or use parseInt()/parseFloat()
Date comparison fails	Comparing string dates instead of DateTime	Convert with .toDateTime() then compare with .diffTo() or .isBetween()

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Variable Availability Matrix

Use this matrix to determine which variables are available in each context.

Variable	Expression Fields	Code Node (JS)	Code Node (Python)	HTTP Request Node
$json / $binary	YES	YES	_json / _binary	YES
$input.item	YES	YES (each-item mode)	_item	YES
$input.all()	YES	YES	_items	YES
$("<Node>").item	YES	NO (use .itemMatching())	NO	YES
$("<Node>").itemMatching()	YES	YES	YES (_("<Node>"))	YES
$itemIndex	YES	NO	NO	YES
$runIndex	YES	YES	YES	YES
$secrets	YES	NO	NO	YES
$env	YES	YES	_env	YES
$vars	YES	YES	_vars	YES
$now / $today	YES	YES	YES	YES
$execution	YES	YES	_execution	YES
$workflow	YES	YES	_workflow	YES
$prevNode	YES	YES	YES	YES
$response	NO	NO	NO	YES
$pageCount	NO	NO	NO	YES
$parameter	YES	YES	YES	YES
$ifEmpty()	YES	YES	YES	YES
$jmespath()	YES	YES	_jmespath()	YES
$getWorkflowStaticData()	YES	YES	_getWorkflowStaticData()	YES
$execution.customData	YES	YES	_execution	YES

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Decision Tree: Expression Not Working

Expression returns unexpected result
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+-- Is the variable available in this context?
|   +-- NO --> Check Variable Availability Matrix above
|   +-- YES --> Continue
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+-- Does the field exist on the item?
|   +-- Check with: {{ Object.keys($json) }}
|   +-- Field missing --> Fix field name or check upstream node output
|   +-- Field exists --> Continue
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+-- Is the value null/undefined/empty?
|   +-- YES --> Use $ifEmpty($json.field, fallback)
|   +-- NO --> Continue
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+-- Is the type correct?
|   +-- String where number expected --> Number($json.field)
|   +-- Number where string expected --> String($json.field)
|   +-- String where date expected --> $json.field.toDateTime()
|   +-- Type is correct --> Continue
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+-- Is item linking the problem?
|   +-- "Paired item not found" error --> See Paired Item Errors below
|   +-- Wrong item data --> Use explicit .first()/.all() instead of .item
|   +-- Correct linking --> Check expression syntax

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Paired Item Error Resolution

When you see "Paired item not found" or get wrong data from $("<Node>").item:

1. Identify the break point — Item linking breaks when a node changes item count (e.g., aggregation, split, filter removes items).
2. Use explicit access instead:
   • $("<Node>").first() — ALWAYS returns first item (safe fallback)
   • $("<Node>").all()[index] — Access by position
   • $("<Node>").itemMatching(currentIndex) — Trace back from current item (preferred in Code node)
3. In Code node — NEVER use $("<Node>").item. ALWAYS use $("<Node>").itemMatching(index).

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JMESPath Parameter Order

n8n uses $jmespath(object, searchString) — this is the OPPOSITE of the JMESPath spec's search(searchString, object).

// CORRECT — object first, search string second
{{ $jmespath($json.data, "[*].name") }}

// WRONG — search string first (JMESPath spec order)
{{ $jmespath("[*].name", $json.data) }}

ALWAYS verify: first argument is the data object, second argument is the query string.

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Static Data Pitfalls

$getWorkflowStaticData() has specific constraints:

• NOT available during manual test execution — returns empty object {}
• ONLY populated when workflow runs via trigger/webhook in production
• Data persists across executions but ONLY after successful completion
• NEVER store large data — keep static data small (IDs, timestamps, counters)
• May be unreliable during high-frequency parallel executions

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Common Error Messages Reference

Error Message	Meaning	Resolution
Expression evaluation error	Generic expression parse failure	Check syntax: matching {{ }}, valid JS
Cannot read properties of undefined (reading 'X')	Accessing property on null/undefined	Add null checks: $json.parent?.child
X is not a function	Wrong method for data type	Check type: strings have .extractEmail(), arrays have .average()
Paired item information is missing	Item link chain broken	Use .first(), .all(), or .itemMatching()
ReferenceError: $itemIndex is not defined	Used in Code node	Use loop index or items.indexOf(item)
ReferenceError: $secrets is not defined	Used in Code node	Use $env or pass via Set node
Invalid left-hand side in assignment	Assignment = inside expression	Expressions are read-only; use Code node for assignment
Unexpected token	Syntax error in expression	Check for unmatched brackets, quotes, or template literals

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Code Node Specific Restrictions

ALWAYS remember these restrictions when writing Code node logic:

1. $itemIndex — NOT available. Use loop index or items.indexOf(item).
2. $secrets — NOT available. Use $env.SECRET_NAME instead.
3. $("<Node>").item — NOT recommended. Use $("<Node>").itemMatching(index).
4. No HTTP requests — use HTTP Request node before/after Code node.
5. No file system access — use Read/Write Files nodes.
6. Python: ALWAYS use bracket notation item["json"]["field"], NEVER dot notation.
7. Python on Cloud: NEVER import external libraries.

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$response Context Restriction

$response is ONLY available in HTTP Request node parameter fields:

Property	Returns	Context
$response.body	Response body object	HTTP Request node ONLY
$response.headers	Response headers	HTTP Request node ONLY
$response.statusCode	HTTP status code	HTTP Request node ONLY
$response.statusMessage	Status message	HTTP Request node ONLY

If you need HTTP response data in other nodes, the HTTP Request node automatically outputs the response as $json for downstream nodes.

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Null/Undefined Handling Strategy

ALWAYS handle potentially missing data with one of these approaches:

1. $ifEmpty(value, fallback) — Best for simple fallback values
2. Optional chaining — $json.parent?.child?.value for nested access
3. Ternary — {{ $json.field ? $json.field : "default" }} for conditional logic
4. IIFE for complex logic:

   {{ (function() {
     const val = $json.field;
     if (val === null || val === undefined) return "N/A";
     return val.toString();
   })() }}
   

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Type Conversion Quick Reference

From	To	Method
String	Number	Number($json.field) or parseInt() / parseFloat()
String	Boolean	$json.field.toBoolean()
String	DateTime	$json.field.toDateTime()
Number	String	String($json.field) or $json.field.format()
Number	Boolean	$json.field.toBoolean() (0 = false, else true)
Number	DateTime	$json.field.toDateTime() (Unix ms or seconds)
Boolean	Number	$json.field.toNumber() (true=1, false=0)
Boolean	String	$json.field.toString()
Array	String	$json.arr.toJsonString()
Object	String	$json.obj.toJsonString()

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Reference Files

• references/methods.md — Expression error types and variable availability details
• references/examples.md — Common expression errors with before/after fixes
• references/anti-patterns.md — Expression anti-patterns to avoid