n8n-expression-testing

# n8n Expression Testing <default_to_action> When testing n8n expressions: 1. VALIDATE syntax before execution 2. TEST with multiple context scenarios 3. CHECK for null/undefined handling 4. VERIFY type safety 5. 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