What does the N8n Expression Testing AI skill do?

n8n expression syntax validation, context-aware testing, common pitfalls detection, and performance optimization. Use when validating n8n expressions and data transformations.

Why use N8n Expression Testing on TypingMind?

Because you install it once and use it with any model. N8n Expression Testing is plain Markdown rather than provider-specific code, so the same skill runs on GPT-5, Claude, Gemini, Grok, or a local model — and you can switch model mid-chat without it breaking. TypingMind runs on your own API keys, so you pay providers directly instead of a per-seat subscription, and your skills and chats stay in your own storage.

How do I install N8n Expression Testing in TypingMind?

Open Plugins → Skills → Install from GitHub in TypingMind and paste https://github.com/proffesor-for-testing/agentic-qe/tree/main/assets/skills/n8n-expression-testing. TypingMind reads its SKILL.md and bundles its files and installs it as a skill you can enable per chat.

Which AI models can use N8n Expression Testing?

Any model you connect in TypingMind. AI skills are plain Markdown instructions rather than provider-specific code, so GPT, Claude, Gemini, Grok, and local models can all load this skill when a request matches it.

How many AI models can I use with N8n Expression Testing?

As many as you like. As long as a model supports skills, you can use N8n Expression Testing with it — GPT, Claude, Gemini, Grok, DeepSeek, Mistral, Llama and more — all on TypingMind with your own API keys.

Is the N8n Expression Testing AI skill free?

Yes. It is published on GitHub by proffesor-for-testing under the MIT license. You only pay your own AI provider for the tokens you use.

An AI skill is a reusable instruction bundle that teaches an AI model how to do one specific task. It follows the open Agent Skills format: a SKILL.md file with a name and description, plus any scripts, templates or reference files the model may need. The model reads the instructions only when your request matches the skill, so an installed skill costs nothing until it is used.

How are AI skills different from plugins or MCP servers?

A plugin or MCP server gives a model new tools to call — code that runs somewhere and returns a result. An AI skill gives the model knowledge and process instead: how to approach a task, which steps to follow, what good output looks like. Skills are plain Markdown, so they need no server, no API key and no runtime, and they work with any model.

N8n Expression Testing by proffesor-for-testing — AI Skill for Any LLM

n8n Expression Testing

<default_to_action> When testing n8n expressions:

VALIDATE syntax before execution
TEST with multiple context scenarios
CHECK for null/undefined handling
VERIFY type safety
SCAN for security vulnerabilities

Quick Expression Checklist:

Valid JavaScript syntax
Context variables properly referenced ($json, $node)
Null-safe access patterns (?., ??)
No dangerous functions (eval, Function)
Efficient for large data sets

Common Pitfalls:

Accessing nested properties without null checks
Type coercion issues
Missing fallback values
Inefficient array operations </default_to_action>

Quick Reference Card

n8n Expression Syntax

Pattern	Example	Description
Basic access	`{{ $json.field }}`	Access JSON field
Nested access	`{{ $json.user.email }}`	Access nested property
Array access	`{{ $json.items[0] }}`	Access array element
Node reference	`{{ $node["Name"].json.id }}`	Access other node's data
Method call	`{{ $json.name.toLowerCase() }}`	Call string method
Conditional	`{{ $json.x ? "yes" : "no" }}`	Ternary expression

Context Variables

Variable	Description	Example
`$json`	Current item data	`{{ $json.email }}`
`$node["Name"]`	Other node's data	`{{ $node["HTTP"].json.body }}`
`$items()`	Multiple items	`{{ $items("Node", 0, 0).json }}`
`$now`	Current timestamp	`{{ $now.toISO() }}`
`$today`	Today's date	`{{ $today }}`
`$runIndex`	Run iteration	`{{ $runIndex }}`
`$workflow`	Workflow info	`{{ $workflow.name }}`

Expression Syntax Patterns

Safe Data Access

javascript
// BAD: Can fail if nested objects are null
{{ $json.user.profile.email }}

// GOOD: Optional chaining with fallback
{{ $json.user?.profile?.email ?? '' }}

// BAD: Array access without bounds check
{{ $json.items[0].name }}

// GOOD: Safe array access
{{ $json.items?.[0]?.name ?? 'No items' }}

Type Conversions

javascript
// String to Number
{{ parseInt($json.quantity, 10) }}
{{ parseFloat($json.price) }}
{{ Number($json.value) }}

// Number to String
{{ String($json.id) }}
{{ $json.amount.toString() }}
{{ $json.count.toFixed(2) }}

// Date handling
{{ new Date($json.timestamp).toISOString() }}
{{ DateTime.fromISO($json.date).toFormat('yyyy-MM-dd') }}

// Boolean conversion
{{ Boolean($json.active) }}
{{ $json.enabled === 'true' }}

String Operations

javascript
// Case conversion
{{ $json.name.toLowerCase() }}
{{ $json.name.toUpperCase() }}
{{ $json.name.charAt(0).toUpperCase() + $json.name.slice(1) }}

// String manipulation
{{ $json.text.trim() }}
{{ $json.text.replace(/\s+/g, ' ') }}
{{ $json.text.substring(0, 100) }}

// Template strings
{{ `Hello, ${$json.firstName} ${$json.lastName}!` }}
{{ `Order #${$json.orderId} - ${$json.status}` }}

Array Operations

javascript
// Mapping
{{ $json.items.map(item => item.name) }}
{{ $json.items.map(item => ({ id: item.id, total: item.price * item.qty })) }}

// Filtering
{{ $json.items.filter(item => item.active) }}
{{ $json.items.filter(item => item.price > 100) }}

// Reducing
{{ $json.items.reduce((sum, item) => sum + item.price, 0) }}
{{ $json.items.reduce((acc, item) => ({ ...acc, [item.id]: item }), {}) }}

// Finding
{{ $json.items.find(item => item.id === $json.targetId) }}
{{ $json.items.findIndex(item => item.name === 'target') }}

// Joining
{{ $json.tags.join(', ') }}
{{ $json.items.map(i => i.name).join(' | ') }}

Validation Patterns

typescript
// Validate expression syntax
function validateExpressionSyntax(expression: string): ValidationResult {
  // Remove n8n template markers
  const code = expression.replace(/\{\{|\}\}/g, '').trim();

  try {
    // Check if valid JavaScript
    new Function(`return (${code})`);
    return { valid: true };
  } catch (error) {
    return {
      valid: false,
      error: error.message,
      suggestion: suggestFix(error.message, code)
    };
  }
}

// Validate context variables
function validateContextVariables(expression: string): string[] {
  const contextVars = ['$json', '$node', '$items', '$now', '$today', '$runIndex', '$workflow'];
  const usedVars = [];
  const invalidVars = [];

  // Find all $ prefixed variables
  const varPattern = /\$\w+/g;
  let match;

  while ((match = varPattern.exec(expression)) !== null) {
    const varName = match[0];
    if (contextVars.some(cv => varName.startsWith(cv))) {
      usedVars.push(varName);
    } else {
      invalidVars.push(varName);
    }
  }

  return { usedVars, invalidVars };
}

// Test expression with sample data
function testExpression(expression: string, context: any): TestResult {
  const code = expression.replace(/\{\{|\}\}/g, '').trim();

  try {
    // Create function with context
    const fn = new Function('$json', '$node', '$items', '$now', '$today',
      `return (${code})`);

    const result = fn(
      context.$json || {},
      context.$node || {},
      context.$items || (() => ({})),
      context.$now || new Date(),
      context.$today || new Date()
    );

    return { success: true, result };
  } catch (error) {
    return { success: false, error: error.message };
  }
}

Common Errors and Fixes

Undefined Property Access

javascript
// ERROR: Cannot read property 'email' of undefined
{{ $json.user.email }}

// FIX 1: Optional chaining
{{ $json.user?.email }}

// FIX 2: With fallback
{{ $json.user?.email ?? 'no-email@example.com' }}

// FIX 3: Conditional
{{ $json.user ? $json.user.email : '' }}

Type Errors

javascript
// ERROR: toLowerCase is not a function (when null)
{{ $json.name.toLowerCase() }}

// FIX: Null check first
{{ $json.name?.toLowerCase() ?? '' }}

// ERROR: toFixed is not a function (string instead of number)
{{ $json.price.toFixed(2) }}

// FIX: Parse as number first
{{ parseFloat($json.price).toFixed(2) }}

// ERROR: map is not a function (not an array)
{{ $json.items.map(i => i.name) }}

// FIX: Ensure array
{{ (Array.isArray($json.items) ? $json.items : []).map(i => i.name) }}

Node Reference Errors

javascript
// ERROR: Node "Previous Node" not found
{{ $node["Previous Node"].json.data }}

// FIX: Use exact node name (case-sensitive)
{{ $node["Previous Node1"].json.data }}

// FIX: Add fallback for safety
{{ $node["Previous Node"]?.json?.data ?? {} }}

Security Patterns

Dangerous Functions to Avoid

javascript
// DANGEROUS: Never use eval
{{ eval($json.code) }}

// DANGEROUS: Dynamic function creation
{{ new Function($json.code)() }}

// DANGEROUS: setTimeout with string
{{ setTimeout($json.code, 1000) }}

// SAFE: Use explicit operations instead
{{ $json.value * 2 }}
{{ JSON.parse($json.jsonString) }}

Input Validation

javascript
// Validate email format
{{ /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test($json.email) ? $json.email : '' }}

// Sanitize for HTML (basic)
{{ $json.text.replace(/[<>&"']/g, c => ({
  '<': '&lt;', '>': '&gt;', '&': '&amp;', '"': '&quot;', "'": '&#39;'
}[c])) }}

// Limit string length
{{ $json.input.substring(0, 1000) }}

// Validate number range
{{ Math.min(Math.max(parseInt($json.value), 0), 100) }}

Performance Optimization

Efficient Array Operations

javascript
// SLOW: Multiple iterations
{{ $json.items.filter(i => i.active).map(i => i.name).join(', ') }}

// FASTER: Single reduce
{{ $json.items.reduce((acc, i) => i.active ? (acc ? `${acc}, ${i.name}` : i.name) : acc, '') }}

// SLOW: Nested loops
{{ $json.items.map(i => $json.categories.find(c => c.id === i.categoryId)) }}

// FASTER: Create lookup map first (in Code node)
const categoryMap = Object.fromEntries($json.categories.map(c => [c.id, c]));
return $json.items.map(i => categoryMap[i.categoryId]);

Avoid in Expressions

javascript
// AVOID: Complex logic in expressions
{{ $json.items.reduce((acc, item) => {
  const category = $json.categories.find(c => c.id === item.catId);
  if (category && category.active) {
    acc.push({ ...item, categoryName: category.name });
  }
  return acc;
}, []) }}

// BETTER: Move to Code node for complex transformations

Testing Patterns

typescript
// Expression test suite
const expressionTests = [
  {
    name: 'Basic property access',
    expression: '{{ $json.name }}',
    context: { $json: { name: 'John' } },
    expected: 'John'
  },
  {
    name: 'Nested with optional chaining',
    expression: '{{ $json.user?.email ?? "default" }}',
    context: { $json: { user: null } },
    expected: 'default'
  },
  {
    name: 'Array mapping',
    expression: '{{ $json.items.map(i => i.id).join(",") }}',
    context: { $json: { items: [{ id: 1 }, { id: 2 }] } },
    expected: '1,2'
  },
  {
    name: 'Conditional expression',
    expression: '{{ $json.score >= 70 ? "Pass" : "Fail" }}',
    context: { $json: { score: 85 } },
    expected: 'Pass'
  },
  {
    name: 'Node reference',
    expression: '{{ $node["Previous"].json.result }}',
    context: { $node: { Previous: { json: { result: 'success' } } } },
    expected: 'success'
  }
];

// Run tests
for (const test of expressionTests) {
  const result = testExpression(test.expression, test.context);
  console.log(`${test.name}: ${result.result === test.expected ? 'PASS' : 'FAIL'}`);
}

Agent Coordination

Memory Namespace


aqe/n8n/expressions/
├── validations/*    - Expression validation results
├── patterns/*       - Discovered expression patterns
├── errors/*         - Common error catalog
└── optimizations/*  - Performance suggestions

Fleet Coordination

typescript
// Coordinate expression validation with workflow testing
await Task("Validate expressions", {
  workflowId: "wf-123",
  validateAll: true,
  testWithSampleData: true
}, "n8n-expression-validator");

Related Skills

n8n-workflow-testing-fundamentals - Workflow testing
n8n-security-testing - Security validation

Remember

n8n expressions are JavaScript-like with special context variables ($json, $node, etc.). Testing requires:

Syntax validation
Context variable verification
Null safety checks
Type compatibility
Security scanning

Key patterns: Use optional chaining (?.) and nullish coalescing (??) for safety. Move complex logic to Code nodes. Always test with edge cases (null, undefined, empty arrays).

Publisher	proffesor-for-testing
Repository	`agentic-qe`
Skill name	`n8n-expression-testing`
Stars	480
Forks	92
Bundled files	3
License	MIT
Links	Skill source Repository Homepage

N8n Expression Testing

Overview

Installation

Install in TypingMind

Install in another agent

Use it in TypingMind

Works with any AI model

Skill instructions

n8n Expression Testing

Quick Reference Card

n8n Expression Syntax

Context Variables

Expression Syntax Patterns

Safe Data Access

Type Conversions

String Operations

Array Operations

Validation Patterns

Common Errors and Fixes

Undefined Property Access

Type Errors

Node Reference Errors

Security Patterns

Dangerous Functions to Avoid

Input Validation

Performance Optimization

Efficient Array Operations

Avoid in Expressions

Testing Patterns

Agent Coordination

Memory Namespace

Fleet Coordination

Related Skills

Remember

Bundled files

Frequently asked questions

Set up your own AI workspace now

Overview

Installation

Install in TypingMind

Install in another agent

Use it in TypingMind

Works with any AI model

Skill instructions

n8n Expression Testing

Quick Reference Card

n8n Expression Syntax

Context Variables

Expression Syntax Patterns

Safe Data Access

Type Conversions

String Operations

Array Operations

Validation Patterns

Common Errors and Fixes

Undefined Property Access

Type Errors

Node Reference Errors

Security Patterns

Dangerous Functions to Avoid

Input Validation

Performance Optimization

Efficient Array Operations

Avoid in Expressions

Testing Patterns

Agent Coordination

Memory Namespace

Fleet Coordination

Related Skills

Remember

Bundled files

Frequently asked questions

Related AI Skills

Set up your own AI workspace now