How to Fix Middleware Class in NestJS

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

NestJS class middleware implements NestMiddleware. Missing @Injectable() decorator or not implementing the interface causes dependency injection failures.

The Problem

Developers working with middleware class 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: MiddlewareClass failed
    at Object.<anonymous> (/app/src/routes.js:15:3)

Quick Fix

1. Apply the correct pattern

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

// Right — correct middleware-class pattern with NestJS
@Controller('class')
export class MiddlewareClassController {
  constructor(private readonly service: MiddlewareClassService) {}

  @Get()
  async getData(): Promise<MiddlewareClassDto> {
    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('Middleware Class failed:', err.message)
    return { success: false, error: err.message }
  }
}
const response = await handleRequestSafe(input)
console.log('Middleware Class status:', response.success)
// Output: Middleware Class status: true

3. Validate inputs and configuration

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

// Right — validate before processing
function safemiddlewareclass(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 = safemiddlewareclass({ value: 'hello' })
console.log('Middleware Class:', result)
// Output: Middleware Class: {result: "HELLO", processed: true}

Prevention

• Always read the NestJS documentation for the correct middleware class API before writing code
• Use TypeScript for better type safety when working with NestJS applications
• Wrap middleware class 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 middleware class issues early
• Use NestJS's built-in error handling as a safety net for unexpected failures

Common Mistakes with middleware class

1. Using return to exit a function early instead of wrapping a pure value in the monad
2. Mixing let bindings with <- bindings in do notation, producing type errors
3. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors

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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