When using an express TypeScript app, errors that are passed through the next() function may not be properly caught by the error handling

Let's take a closer look at this code snippet:

import express from 'express';

// Custom error handling for non-existing routes
app.use((req: Request, res: Response, next: NextFunction) => {
  return next(new Error('Test error));
});

// Global error handling example
app.use((error: any, req: Request, res: Response) => {
  res
    .status(500)
    .json({ 'Server error' });
});

An issue arises when an error is triggered within the missing routes middleware. This prevents the error from being caught by the global error handling middleware, resulting in HTML responses instead of JSON being sent back to the client.

Answer ā„–1

In Express, the error handler function stands out as a special type of middleware designed to capture and handle errors effectively. This middleware should be equipped with precisely four parameters:

  1. The "error" parameter which represents an instance of any error or a specific type of error that needs to be caught,
  2. The request object,
  3. The response object,
  4. The "next" function - essential for declaring multiple error handlers and maintaining the flow by calling "next".

All four parameters are mandatory for proper function.

// Error handling
// Example implementation
app.use((error: any, req: Request, res: Response, next: NextFunction) => 
{
  res
    .status(500)
    .json({ 'Server error' });
});

Furthermore, it is crucial to position the error handler middleware at the end after all route handlers have been declared.

Answer ā„–2

To tackle this issue, I successfully resolved it by introducing the next parameter into the error handling middleware:

// Error handling
app.use((error: any, req: Request, res: Response, next: NextFunction) => {
  res
    .status(500)
    .json({ 'Server error' });
});

I am intrigued as to what could have caused this change in behavior, considering that this middleware function was functioning properly prior to the addition of TypeScript.

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