How to Fix Filter Exception in NestJS

In this tutorial, you'll learn about How to Fix Filter Exception in NestJS. We cover key concepts, practical examples, and best practices.

NestJS exception filters catch thrown exceptions. Without filters, unhandled exceptions return default 500 responses instead of structured error objects.

The Problem

Developers working with filter exception in NestJS often encounter runtime errors, unexpected behavior, and production failures. These issues commonly stem from incorrect API usage, missing configuration, wrong middleware ordering, or misunderstanding the framework's design patterns.

Error: FilterException failed
    at Object.<anonymous> (/app/src/routes.js:15:3)

Quick Fix

1. Apply the correct pattern

// Wrong — incorrect filter-exception usage in NestJS
@Controller()
export class AppController {
  @Get()
  getData() {
    // Missing return or wrong structure
  }
}

// Right — correct filter-exception pattern with NestJS
@Controller('exception')
export class FilterExceptionController {
  constructor(private readonly service: FilterExceptionService) {}

  @Get()
  async getData(): Promise<FilterExceptionDto> {
    try {
      const data = await this.service.fetchData()
      if (!data) throw new NotFoundException('No data found')
      return { success: true, data }
    } catch (err) {
      throw new HttpException(err.message, err.status || 500)
    }
  }
}

2. Handle async errors properly

// Wrong — uncaught async rejection
async function handleRequest(data) {
  const result = await processData(data)
  return result
}
// If processData throws, the error is unhandled

// Right — wrap async operations in try-catch
async function handleRequestSafe(data) {
  try {
    if (!data) throw new Error('Input required')
    const result = await processData(data)
    if (!result) throw new Error('Processing returned empty')
    return { success: true, data: result }
  } catch (err) {
    console.error('Filter Exception failed:', err.message)
    return { success: false, error: err.message }
  }
}
const response = await handleRequestSafe(input)
console.log('Filter Exception status:', response.success)
// Output: Filter Exception status: true

3. Validate inputs and configuration

// Wrong — assuming inputs are always valid
function processfilterexception(input) {
  return input.value.toUpperCase()
}

// Right — validate before processing
function safefilterexception(input) {
  if (!input || typeof input !== 'object') {
    return { error: 'Input must be an object' }
  }
  if (!input.value || typeof input.value !== 'string') {
    return { error: 'Input.value must be a string' }
  }
  return { result: input.value.toUpperCase(), processed: true }
}
const result = safefilterexception({ value: 'hello' })
console.log('Filter Exception:', result)
// Output: Filter Exception: {result: "HELLO", processed: true}

Prevention

• Always read the NestJS documentation for the correct filter exception API before writing code
• Use TypeScript for better type safety when working with NestJS applications
• Wrap filter exception operations in try-catch blocks to handle runtime errors gracefully
• Write integration tests that cover request-response cycles for your API
• Follow DodaTech coding standards for consistent patterns across your codebase
• Monitor production with structured logging to catch filter exception issues early
• Use NestJS's built-in error handling as a safety net for unexpected failures

Common Mistakes with filter exception

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world NESTJS code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

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