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# Functions
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Serverless function management including creation, local development, testing, and deployment of Netlify Functions with support for multiple languages and frameworks.
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## Capabilities
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### Function Creation
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Create new serverless functions from templates with language and framework support.
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```typescript { .api }
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/**
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 * Create a new serverless function from template
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 * Command: netlify functions:create [name] [options]
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 */
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interface FunctionCreateOptions {
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  /** Function name */
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  name?: string;
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  /** Template URL or repository */
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  url?: string;
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  /** Programming language (javascript, typescript, go, etc.) */
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  language?: string;
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  /** Disable network features during creation */
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  offline?: boolean;
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}
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```
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**Usage Examples:**
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```bash
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# Create function with interactive prompts
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netlify functions:create
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# Create named function
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netlify functions:create hello-world
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# Create function with specific language
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netlify functions:create api-handler --language typescript
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# Create function from custom template
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netlify functions:create custom-func --url https://github.com/user/template
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# Create function offline
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netlify functions:create --offline
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```
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### Function Development and Testing
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Local development and testing tools for serverless functions.
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```typescript { .api }
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/**
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 * Build functions locally
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 * Command: netlify functions:build [options]
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 */
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interface FunctionBuildOptions {
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  /** Functions directory */
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  functions?: string;
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  /** Source directory for functions */
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  src?: string;
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}
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/**
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 * Invoke/test functions locally
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 * Command: netlify functions:invoke [name] [options]
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 */
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interface FunctionInvokeOptions {
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  /** Function name to invoke */
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  name?: string;
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  /** Functions folder */
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  functions?: string;
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  /** Query string parameters */
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  querystring?: string;
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  /** POST payload data */
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  payload?: string;
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  /** Simulate Netlify Identity JWT token */
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  identity?: boolean;
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  /** Simulate unauthenticated request */
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  noIdentity?: boolean;
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  /** Netlify dev server port */
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  port?: number;
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  /** Disable network features */
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  offline?: boolean;
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}
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/**
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 * List local functions
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 * Command: netlify functions:list [options]
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 */
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interface FunctionListOptions {
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  /** Functions directory */
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  functions?: string;
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  /** Output as JSON */
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  json?: boolean;
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}
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/**
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 * Serve functions locally
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 * Command: netlify functions:serve [options]
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 */
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interface FunctionServeOptions {
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  /** Functions directory */
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  functions?: string;
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  /** Functions server port */
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  port?: number;
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  /** Disable network features */
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  offline?: boolean;
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}
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```
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**Usage Examples:**
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```bash
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# Build functions
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netlify functions:build
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# Build from custom source directory
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netlify functions:build --src src/lambda --functions dist/functions
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# List all functions
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netlify functions:list
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# List functions as JSON
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netlify functions:list --json
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# Invoke function locally
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netlify functions:invoke hello-world
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# Invoke with query parameters
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netlify functions:invoke api --querystring "id=123&type=user"
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# Invoke with POST data
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netlify functions:invoke create-user --payload '{"name":"John","email":"john@example.com"}'
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# Invoke with Identity simulation
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netlify functions:invoke protected-route --identity
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# Serve functions on custom port
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netlify functions:serve --port 9999
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```
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### Function Templates and Languages
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Supported languages and template system for function creation:
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```typescript { .api }
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/**
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 * Supported function languages and runtimes
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 */
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type FunctionLanguage = 
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  | 'javascript'
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  | 'typescript' 
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  | 'go'
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  | 'rust'
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  | 'python';
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/**
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 * Function template configuration
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 */
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interface FunctionTemplate {
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  /** Template name */
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  name: string;
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  /** Template description */
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  description: string;
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  /** Supported language */
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  language: FunctionLanguage;
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  /** Template repository URL */
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  url: string;
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  /** Required dependencies */
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  dependencies?: string[];
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  /** Environment variables needed */
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  envVars?: string[];
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  /** Template category */
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  category: 'api' | 'scheduled' | 'auth' | 'form' | 'webhook' | 'utility';
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}
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/**
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 * Popular built-in templates
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 */
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interface BuiltInTemplates {
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  'hello-world': FunctionTemplate;
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  'fetch-api': FunctionTemplate;
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  'send-email': FunctionTemplate;
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  'stripe-webhook': FunctionTemplate;
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  'auth-callback': FunctionTemplate;
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  'scheduled-task': FunctionTemplate;
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  'form-handler': FunctionTemplate;
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  'image-processing': FunctionTemplate;
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  'database-query': FunctionTemplate;
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}
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```
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### Function Configuration and Environment
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Function configuration options and environment setup:
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```typescript { .api }
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/**
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 * Function configuration in netlify.toml
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 */
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interface FunctionConfig {
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  /** Function-specific configuration */
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  functions: {
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    /** Functions directory */
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    directory: string;
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    /** Node.js version */
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    node_bundler?: 'esbuild' | 'zisi';
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    /** External node modules to include */
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    external_node_modules?: string[];
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    /** Included files pattern */
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    included_files?: string[];
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    /** Ignore patterns for function bundling */
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    ignore?: string[];
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  };
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  /** Individual function settings */
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  [functionName: string]: {
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    /** Runtime for this function */
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    runtime?: string;
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    /** Timeout in seconds (max 900) */
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    timeout?: number;
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    /** Memory allocation in MB */
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    memory?: number;
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    /** Environment variables */
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    environment?: Record<string, string>;
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    /** Schedule for background functions */
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    schedule?: string;
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  };
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}
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/**
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 * Function runtime environment
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 */
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interface FunctionEnvironment {
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  /** Netlify-provided environment variables */
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  netlifyVars: {
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    NETLIFY_DEV: string;
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    URL: string;
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    DEPLOY_URL: string;
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    CONTEXT: string;
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    BRANCH: string;
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    HEAD: string;
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    COMMIT_REF: string;
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  };
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  /** User-defined environment variables */
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  userVars: Record<string, string>;
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  /** Function-specific variables */
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  functionVars: Record<string, string>;
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}
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```
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### Function Event and Context Objects
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Structure of event and context objects passed to functions:
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```typescript { .api }
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/**
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 * Netlify Function event object
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 */
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interface NetlifyEvent {
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  /** HTTP method */
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  httpMethod: 'GET' | 'POST' | 'PUT' | 'DELETE' | 'PATCH' | 'OPTIONS' | 'HEAD';
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  /** Request path */
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  path: string;
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  /** Query string parameters */
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  queryStringParameters: Record<string, string> | null;
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  /** Multi-value query string parameters */
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  multiValueQueryStringParameters: Record<string, string[]> | null;
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  /** Request headers */
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  headers: Record<string, string>;
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  /** Multi-value headers */
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  multiValueHeaders: Record<string, string[]>;
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  /** Request body (base64 encoded if binary) */
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  body: string | null;
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  /** Whether body is base64 encoded */
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  isBase64Encoded: boolean;
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}
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/**
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 * Netlify Function context object
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 */
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interface NetlifyContext {
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  /** Invocation ID */
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  awsRequestId: string;
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  /** Function name */
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  functionName: string;
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  /** Function version */
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  functionVersion: string;
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  /** Remaining time in milliseconds */
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  getRemainingTimeInMillis(): number;
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  /** CloudWatch log group */
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  logGroupName: string;
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  /** CloudWatch log stream */
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  logStreamName: string;
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  /** Memory limit in MB */
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  memoryLimitInMB: string;
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  /** Identity information if authenticated */
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  identity?: {
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    url: string;
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    token: string;
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    claims: Record<string, any>;
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  };
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  /** Client context for mobile apps */
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  clientContext?: {
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    client: {
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      installation_id: string;
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      app_title: string;
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      app_version_name: string;
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      app_version_code: string;
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      app_package_name: string;
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    };
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    env: Record<string, string>;
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  };
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}
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/**
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 * Function response format
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 */
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interface NetlifyResponse {
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  /** HTTP status code */
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  statusCode: number;
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  /** Response headers */
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  headers?: Record<string, string>;
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  /** Multi-value headers */
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  multiValueHeaders?: Record<string, string[]>;
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  /** Response body */
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  body: string;
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  /** Whether body is base64 encoded */
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  isBase64Encoded?: boolean;
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}
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/**
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 * Function handler signature
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 */
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type NetlifyHandler = (
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  event: NetlifyEvent,
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  context: NetlifyContext
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) => Promise<NetlifyResponse> | NetlifyResponse;
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```
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### Function Types and Patterns
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Different types of functions and common patterns:
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```typescript { .api }
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/**
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 * API endpoint function pattern
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 */
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interface ApiFunction {
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  handler: NetlifyHandler;
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  /** HTTP methods this function handles */
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  methods: ('GET' | 'POST' | 'PUT' | 'DELETE' | 'PATCH')[];
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  /** Path pattern */
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  path: string;
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  /** Authentication required */
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  requiresAuth?: boolean;
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  /** Rate limiting configuration */
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  rateLimit?: {
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    windowMs: number;
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    maxRequests: number;
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  };
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}
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/**
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 * Background/scheduled function pattern
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 */
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interface BackgroundFunction {
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  handler: NetlifyHandler;
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  /** Cron schedule expression */
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  schedule: string;
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  /** Timezone for schedule */
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  timezone?: string;
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  /** Function description */
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  description: string;
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}
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/**
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 * Form handler function pattern
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 */
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interface FormHandler {
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  handler: NetlifyHandler;
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  /** Form name to handle */
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  formName: string;
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  /** Spam protection */
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  spamProtection?: boolean;
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  /** Success redirect URL */
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  successRedirect?: string;
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  /** Error redirect URL */
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  errorRedirect?: string;
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}
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/**
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 * Webhook function pattern
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 */
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interface WebhookFunction {
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  handler: NetlifyHandler;
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  /** Webhook provider */
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  provider: 'stripe' | 'github' | 'slack' | 'zapier' | 'custom';
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  /** Signature verification */
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  verifySignature?: boolean;
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  /** Webhook secret for verification */
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  secret?: string;
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}
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```
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### Function Development Workflow
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Typical development workflow for Netlify Functions:
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```typescript { .api }
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/**
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 * Function development lifecycle
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 */
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interface FunctionWorkflow {
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  /** Development phase */
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  development: {
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    /** Create function from template */
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    create: () => Promise<void>;
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    /** Local development server */
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    serve: () => Promise<void>;
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    /** Test function locally */
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    test: () => Promise<void>;
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    /** Debug with logs */
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    debug: () => Promise<void>;
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  };
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  /** Testing phase */
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  testing: {
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    /** Unit testing */
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    unit: () => Promise<void>;
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    /** Integration testing */
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    integration: () => Promise<void>;
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    /** Load testing */
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    load: () => Promise<void>;
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  };
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  /** Deployment phase */
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  deployment: {
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    /** Build for production */
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    build: () => Promise<void>;
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    /** Deploy to staging */
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    deployStaging: () => Promise<void>;
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    /** Deploy to production */
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    deployProduction: () => Promise<void>;
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  };
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  /** Monitoring phase */
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  monitoring: {
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    /** View function logs */
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    logs: () => Promise<void>;
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    /** Monitor performance */
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    performance: () => Promise<void>;
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    /** Error tracking */
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    errors: () => Promise<void>;
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  };
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}
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```
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### Function Error Handling
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Error handling patterns and debugging for functions:
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```typescript { .api }
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/**
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 * Function error types
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 */
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interface FunctionError {
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  /** Error type */
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  type: 'syntax' | 'runtime' | 'timeout' | 'memory' | 'network' | 'user';
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  /** Error message */
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  message: string;
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  /** Stack trace */
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  stack?: string;
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  /** Request ID for debugging */
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  requestId: string;
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  /** Function name */
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  functionName: string;
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  /** Timestamp */
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  timestamp: Date;
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}
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/**
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 * Error response helpers
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 */
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interface ErrorHelpers {
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  /** Create 400 Bad Request response */
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  badRequest: (message: string) => NetlifyResponse;
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  /** Create 401 Unauthorized response */
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  unauthorized: (message?: string) => NetlifyResponse;
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  /** Create 403 Forbidden response */
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  forbidden: (message?: string) => NetlifyResponse;
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  /** Create 404 Not Found response */
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  notFound: (message?: string) => NetlifyResponse;
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  /** Create 500 Internal Server Error response */
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  serverError: (message?: string) => NetlifyResponse;
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  /** Create custom error response */
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  customError: (statusCode: number, message: string) => NetlifyResponse;
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}
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```