Error Handling — Handling Backend Failures Properly

import express from 'express';
const app = express();
app.use(express.json());

// 1. In Express 5, async route errors are automatically caught!
// You DO NOT need try/catch blocks or external wrapper packages!
app.get('/api/tasks/:id', async (req, res) => {
  const task = await taskService.findTask(req.params.id);
  if (!task) {
    // Throwing an error automatically forwards to the 4-parameter error middleware!
    const err = new Error('Task not found');
    err.statusCode = 404;
    err.code = 'TASK_NOT_FOUND';
    throw err;
  }
  res.json({ task });
});

// 2. Error-handling middleware MUST be defined LAST (after all routes)
// MUST have exactly 4 arguments: (err, req, res, next)
app.use((err, req, res, next) => {
  // Log full stack trace privately to server console / logger
  console.error('[INTERNAL LOG]', err.stack);

  const status = err.statusCode || 500;
  res.status(status).json({
    error: {
      code: err.code || 'INTERNAL_SERVER_ERROR',
      message: status === 500 ? 'An unexpected error occurred.' : err.message
    }
  });
});

from fastapi import FastAPI, HTTPException, Request, status
from fastapi.responses import JSONResponse
from starlette.exceptions import HTTPException as StarletteHTTPException

app = FastAPI()

# 1. Path Operation: Use 'raise' (NEVER 'return') HTTPException
@app.get("/api/tasks/{task_id}")
async def get_task(task_id: int):
    task = await get_task_from_db(task_id)
    if not task:
        # Raising immediately halts execution and triggers exception handling
        raise HTTPException(
            status_code=status.HTTP_404_NOT_FOUND,
            detail={"code": "TASK_NOT_FOUND", "message": f"Task {task_id} not found"}
        )
    return {"task": task}

# 2. Global Exception Handler: Formats all HTTPExceptions uniformly
@app.exception_handler(StarletteHTTPException)
async def custom_http_exception_handler(request: Request, exc: StarletteHTTPException):
    return JSONResponse(
        status_code=exc.status_code,
        content={"error": exc.detail}
    )

# 3. Catch-all for unexpected 500 server crashes
@app.exception_handler(Exception)
async def unhandled_exception_handler(request: Request, exc: Exception):
    # Log private stack trace to server logs
    print(f"[INTERNAL ERROR] {repr(exc)}")
    return JSONResponse(
        status_code=500,
        content={"error": {"code": "INTERNAL_SERVER_ERROR", "message": "An unexpected error occurred."}}
    )