Expressions

Expressions allow you to dynamically transform and reference data in n8n workflows. They enable you to access data from previous nodes, use built-in functions, and create complex data transformations.

Expression Basics

Expressions in n8n are wrapped in double curly braces and start with an equals sign:

={{$json.fieldName}}
={{$node["NodeName"].json.data}}
={{new Date().toISOString()}}

When to Use Expressions

Expressions can be used in:

Node parameters (URLs, headers, body data)
Conditional logic (IF nodes, Switch nodes)
Data transformation (Set nodes, Code nodes)
Filter operations
Any string-type parameter

Expression Evaluation

The Expression class handles all expression evaluation in n8n:

// From expression.ts:180-181
export class Expression {
  constructor(private readonly workflow: Workflow) {}

  // Initialize secure sandbox environment
  static initializeGlobalContext(data: IDataObject) {
    // Denylist dangerous globals
    data.document = {};
    data.window = {};
    data.eval = {};
    data.Function = {};
    
    // Allowlist safe utilities
    data.Date = Date;
    data.DateTime = DateTime;    // Luxon DateTime
    data.Math = Math;
    data.JSON = JSON;
    data.Object = createSafeObject();  // Sanitized Object
  }
}

Expressions run in a sandboxed environment that blocks dangerous operations like eval(), Function(), fetch(), and access to window or document objects.

Data Access Patterns

Current Node Data

JSON Data
Binary Data
Item Index
Node Input

Access JSON data from the current item:

// Current item's data
{{$json.fieldName}}
{{$json.user.email}}
{{$json.items[0].id}}

// Nested access
{{$json.response.data.users[0].name}}

Access binary file data:

// Binary file properties
{{$binary.data.fileName}}
{{$binary.data.mimeType}}
{{$binary.data.fileSize}}

// Check if binary data exists
{{$binary.data !== undefined}}

Get the current item’s index:

// Zero-based index
{{$itemIndex}}

// Use in expressions
{{"Item " + ($itemIndex + 1)}}

Access all input data:

// All input items
{{$input.all()}}

// First item
{{$input.first()}}

// Last item
{{$input.last()}}

// Specific item
{{$input.item.json.fieldName}}

Previous Node Data

// Access data from a specific node
{{$node["HTTP Request"].json.userId}}
{{$node["Webhook"].json.body.email}}

// Access specific item from a node
{{$node["HTTP Request"].json[0].id}}

// Check if node has data
{{$node["HTTP Request"].json !== undefined}}

Multiple Items

When processing multiple items:

// Map over all items
{{$input.all().map(item => item.json.name)}}

// Filter items
{{$input.all().filter(item => item.json.age > 18)}}

// Reduce items
{{$input.all().reduce((sum, item) => sum + item.json.amount, 0)}}

Built-in Functions and Variables

Date and Time

// JavaScript Date
{{new Date()}}
{{new Date().toISOString()}}
{{Date.now()}}

// Luxon DateTime (recommended)
{{DateTime.now()}}
{{DateTime.now().toISO()}}
{{DateTime.now().toFormat('yyyy-MM-dd')}}

Math Operations

// Basic math
{{Math.round($json.price * 1.2)}}
{{Math.floor($json.value)}}
{{Math.max(...$input.all().map(i => i.json.score))}}

// Random numbers
{{Math.random()}}
{{Math.floor(Math.random() * 100)}}

String Operations

// String methods
{{$json.name.toUpperCase()}}
{{$json.email.toLowerCase()}}
{{$json.text.trim()}}
{{$json.description.substring(0, 100)}}

// Template literals
{{`Hello ${$json.name}, your order #${$json.orderId} is ready`}}

// String concatenation
{{$json.firstName + " " + $json.lastName}}

// Regular expressions
{{/\d{3}-\d{4}/.test($json.phone)}}
{{$json.text.replace(/\s+/g, '-')}}

Array Operations

// Array methods
{{$json.items.length}}
{{$json.items.map(i => i.id)}}
{{$json.items.filter(i => i.active)}}
{{$json.items.find(i => i.id === 123)}}
{{$json.items.some(i => i.quantity > 0)}}
{{$json.items.every(i => i.validated)}}

// Array manipulation
{{$json.items.slice(0, 5)}}
{{$json.items.concat($json.moreItems)}}
{{[...$json.items, newItem]}}

Object Operations

// Object methods
{{Object.keys($json)}}
{{Object.values($json)}}
{{Object.entries($json)}}

// Object manipulation
{{Object.assign({}, $json, {newField: 'value'})}}
{{{...$json, updatedField: 'new value'}}}

// Check properties
{{$json.hasOwnProperty('fieldName')}}
{{'fieldName' in $json}}

Workflow Data Proxy

The WorkflowDataProxy class provides access to workflow execution data:

// Available in expressions
const dataProxy = {
  $json: currentItem.json,
  $binary: currentItem.binary,
  $itemIndex: currentItemIndex,
  $node: nodeDataProxy,
  $input: inputProxy,
  $parameter: nodeParameters,
  $workflow: workflowMetadata,
  $execution: executionMetadata,
  $vars: environmentVariables,
  $env: environmentVariables  // Alias for $vars
};

Workflow Metadata

// Workflow information
{{$workflow.id}}
{{$workflow.name}}
{{$workflow.active}}

// Execution information
{{$execution.id}}
{{$execution.mode}}  // 'manual', 'trigger', 'webhook'
{{$execution.resumeUrl}}  // For wait nodes

Environment Variables

// Access environment variables
{{$env.API_KEY}}
{{$vars.DATABASE_URL}}

// Use in configurations
{{
  url: $env.API_ENDPOINT,
  headers: {
    'Authorization': `Bearer ${$env.API_TOKEN}`
  }
}}

Expression Security

Sandbox Protection

The expression sandbox prevents malicious code execution:

// From expression.ts:186-235
static initializeGlobalContext(data: IDataObject) {
  // Blocked: Remote code execution
  data.eval = {};
  data.Function = {};
  data.setTimeout = {};
  data.setInterval = {};

  // Blocked: Network requests
  data.fetch = {};
  data.XMLHttpRequest = {};

  // Blocked: Control flow
  data.Promise = {};
  data.Generator = {};
  data.AsyncFunction = {};

  // Blocked: Low-level access
  data.WebAssembly = {};
  data.Reflect = {};
  data.Proxy = {};

  // Allowed: Safe utilities
  data.Date = Date;
  data.DateTime = DateTime;
  data.Math = Math;
  data.JSON = JSON;
  data.Object = createSafeObject();  // Sanitized version
}

The safe Object wrapper blocks dangerous methods like defineProperty, setPrototypeOf, and __defineGetter__ that could bypass security checks.

Error Handling

Expressions can throw specific errors:

// From expression.ts:39-51
const isSyntaxError = (error: unknown): error is SyntaxError =>
  error instanceof SyntaxError || 
  (error instanceof Error && error.name === 'SyntaxError');

const isExpressionError = (error: unknown): error is ExpressionError =>
  error instanceof ExpressionError || 
  error instanceof ExpressionExtensionError;

const isTypeError = (error: unknown): error is TypeError =>
  error instanceof TypeError || 
  (error instanceof Error && error.name === 'TypeError');

Advanced Patterns

Conditional Expressions

// Ternary operator
{{$json.status === 'active' ? 'Active User' : 'Inactive User'}}

// Nullish coalescing
{{$json.description ?? 'No description provided'}}

// Logical OR for defaults
{{$json.count || 0}}
{{$json.name || 'Unknown'}}

// Complex conditions
{{
  $json.score >= 90 ? 'A' :
  $json.score >= 80 ? 'B' :
  $json.score >= 70 ? 'C' : 'F'
}}

Data Transformation

// Transform array of objects
{{
  $json.users.map(user => ({
    id: user.id,
    fullName: `${user.firstName} ${user.lastName}`,
    email: user.email.toLowerCase()
  }))
}}

// Aggregate data
{{
  $input.all().reduce((acc, item) => {
    const category = item.json.category;
    acc[category] = (acc[category] || 0) + item.json.amount;
    return acc;
  }, {})
}}

// Flatten nested arrays
{{$json.items.flatMap(item => item.tags)}}

Dynamic Node References

Expressions can reference nodes dynamically:

// Dynamic node name
{{$node[$json.sourceNode].json.data}}

// Conditional node reference
{{
  $json.useCache 
    ? $node["Cache"].json.result 
    : $node["API"].json.result
}}

Expression Extensions

Custom extensions can be added to expressions:

// From extensions/expression-extension.ts
import { extend, extendOptional } from './extensions';
import { extendSyntax } from './extensions/expression-extension';
import { extendedFunctions } from './extensions/extended-functions';

// Extend expression capabilities
extend(dataProxy, functions, methods);

Expression Best Practices

Keep expressions simple: Complex logic belongs in Code nodes
Use descriptive variable names: When using intermediate variables
Handle null/undefined: Always check for data existence
Use type-safe operations: Be aware of data types
Avoid side effects: Expressions should be pure functions
Test with sample data: Use the expression editor’s test mode
Document complex expressions: Add comments in Code nodes

Expression Editor Tips:

Press Ctrl+Space for autocomplete
Hover over variables to see their values
Use the “Test with sample data” feature
Check the expression syntax highlighting for errors

Common Patterns

API Request with Dynamic Parameters

// URL with query parameters
{{$json.baseUrl + '?page=' + $json.page + '&limit=' + $json.limit}}

// Or using template literal
{{`${$json.baseUrl}?page=${$json.page}&limit=${$json.limit}`}}

Data Validation

// Check required fields
{{
  $json.email && 
  $json.name && 
  $json.email.includes('@')
}}

// Validate number range
{{$json.age >= 18 && $json.age <= 120}}

// Validate date
{{DateTime.fromISO($json.date).isValid}}

Error Messages

// Descriptive error message
{{
  `Failed to process item ${$itemIndex + 1}: ` +
  `Missing required field '${$json.missingField}'`
}}

Getting Started

Self-Hosting

Building Workflows

AI Workflows

Node Development

CLI Reference

Expressions

Expressions

Expression Basics

When to Use Expressions

Expression Evaluation

Data Access Patterns

Current Node Data

Previous Node Data

Multiple Items

Built-in Functions and Variables

Date and Time

Math Operations

String Operations

Array Operations

Object Operations

Workflow Data Proxy

Workflow Metadata

Environment Variables

Expression Security

Sandbox Protection

Error Handling

Advanced Patterns

Conditional Expressions

Data Transformation

Dynamic Node References

Expression Extensions

Expression Best Practices

Common Patterns

API Request with Dynamic Parameters

Data Validation

Error Messages

Next Steps

Error Handling

Execution Modes

Getting Started

Self-Hosting

Building Workflows

AI Workflows

Node Development

CLI Reference

​Expressions

​Expression Basics

​When to Use Expressions

​Expression Evaluation

​Data Access Patterns

​Current Node Data

​Previous Node Data

​Multiple Items

​Built-in Functions and Variables

​Date and Time

​Math Operations

​String Operations

​Array Operations

​Object Operations

​Workflow Data Proxy

​Workflow Metadata

​Environment Variables

​Expression Security

​Sandbox Protection

​Error Handling

​Advanced Patterns

​Conditional Expressions

​Data Transformation

​Dynamic Node References

​Expression Extensions

​Expression Best Practices

​Common Patterns

​API Request with Dynamic Parameters

​Data Validation

​Error Messages

​Next Steps

Error Handling

Execution Modes

Expressions

Expression Basics

When to Use Expressions

Expression Evaluation

Data Access Patterns

Current Node Data

Previous Node Data

Multiple Items

Built-in Functions and Variables

Date and Time

Math Operations

String Operations

Array Operations

Object Operations

Workflow Data Proxy

Workflow Metadata

Environment Variables

Expression Security

Sandbox Protection

Error Handling

Advanced Patterns

Conditional Expressions

Data Transformation

Dynamic Node References

Expression Extensions

Expression Best Practices

Common Patterns

API Request with Dynamic Parameters

Data Validation

Error Messages

Next Steps