Express - lesson 7 | HackIndia Skills

Express - lesson 7 | HackIndia Skills | HackIndia

Example

Route to handler

In Express 5, async route handlers that reject or throw are passed to error middleware. You do not need the old Express 4 wrapper pattern for every async route.

Use next(error) when you create an error yourself, such as when a resource is not found.

import express from 'express';

const app = express();
app.use(express.json());

const students = [
  { id: '1', name: 'Asha', college: 'VIT' }
];

app.get('/students/:id', async (req, res, next) => {
  const student = students.find((item) => item.id === req.params.id);

  if (!student) {
    const error = new Error('Student not found');
    error.status = 404;
    return next(error);
  }

  res.json({ data: student });
});

app.post('/students', async (req, res) => {
  if (!req.body?.name) {
    const error = new Error('Name is required');
    error.status = 400;
    throw error;
  }

  res.status(201).json({ data: req.body });
});

app.use((err, req, res, next) => {
  if (res.headersSent) {
    return next(err);
  }

  const status = err.status || err.statusCode || 500;

  res.status(status).json({
    error: {
      message: status === 500 ? 'Internal server error' : err.message,
      status
    }
  });
});

app.listen(process.env.PORT || 3000);

Common mistake

Do not leak stacks

Stack traces help you debug, but they should not be shown to users. They can reveal file names, folders, and internal code details.

For your project, return a safe message for 500 errors. Log the full error in your terminal while developing if you need it.

app.use((err, req, res, next) => {
  console.error(err);

  const status = err.status || err.statusCode || 500;
  const message = status === 500 ? 'Internal server error' : err.message;

  res.status(status).json({ error: { message, status } });
});

Tip

Bad JSON is covered

If express.json() cannot parse the request body, it creates a parse error. These errors commonly carry a status like 400.

That means your central handler can also format malformed JSON errors. Test it by sending broken JSON in Postman, Thunder Client, or curl.

curl -X POST http://localhost:3000/students \
  -H "Content-Type: application/json" \
  -d '{ "name": '

Common mistake

Order matters

If your error handler is above your routes, it will not catch route errors. Express runs middleware in order.

Keep normal middleware first, then routes, then the final error handler.

app.use(express.json());

app.use('/students', studentsRouter);
app.use('/projects', projectsRouter);

app.use((err, req, res, next) => {
  // final error response here
});

Handle Errors in One Place

One exit for errors

Why this matters

Route to handler

Add the handler

Do not leak stacks

Bad JSON is covered

Order matters

Do this now

Quick check