Synchronous error handling in Express

Introduction

Synchronous error handling helps catch mistakes that happen right away in your code. It stops the app from crashing and shows a clear message.

When you want to catch errors from code that runs immediately inside a route.

When you need to handle mistakes like missing variables or wrong data types in your request.

When you want to send a friendly error message back to the user if something goes wrong.

When debugging your app to find where it breaks in synchronous code.

When you want to keep your server running even if a route has a problem.

Syntax

Express

app.get('/route', (req, res, next) => {
  try {
    // code that might throw an error
  } catch (err) {
    next(err); // pass error to Express error handler
  }
});

Use try...catch to catch errors in synchronous code.

Call next(err) to send the error to Express's error handler middleware.

Examples

This example checks if a name is given in the query. If not, it throws an error caught by try...catch.

Express

app.get('/hello', (req, res, next) => {
  try {
    if (!req.query.name) throw new Error('Name is required');
    res.send(`Hello, ${req.query.name}!`);
  } catch (err) {
    next(err);
  }
});

This example handles division and throws an error if dividing by zero.

Express

app.get('/divide', (req, res, next) => {
  try {
    const a = Number(req.query.a);
    const b = Number(req.query.b);
    if (b === 0) throw new Error('Cannot divide by zero');
    res.send(`Result: ${a / b}`);
  } catch (err) {
    next(err);
  }
});

Sample Program

This Express app has a route /greet that requires a name query parameter. If missing, it throws an error caught by try...catch and passed to the error handler middleware. The error handler sends a 400 status with the error message as JSON.

Express

import express from 'express';
const app = express();

app.get('/greet', (req, res, next) => {
  try {
    const name = req.query.name;
    if (!name) throw new Error('Missing name parameter');
    res.send(`Hello, ${name}!`);
  } catch (err) {
    next(err);
  }
});

// Error handler middleware
app.use((err, req, res, next) => {
  res.status(400).send({ error: err.message });
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Always use next(err) inside catch to let Express handle the error properly.

Errors thrown outside try...catch in synchronous code will crash the app if not handled.

For asynchronous code, use different error handling methods (like async/await with try...catch or error middleware).

Summary

Synchronous errors happen immediately and can be caught with try...catch.

Use next(err) to pass errors to Express error handlers.

This keeps your app stable and user-friendly by showing clear error messages.

Practice

1. In Express, how do you handle synchronous errors inside a route handler?

easy

A. Use setTimeout to catch errors asynchronously

B. Use async/await without try...catch

C. Ignore errors and let the server crash

D. Use a try...catch block and call next(error) inside catch

Solution

1. Step 1: Understand synchronous error handling

   Synchronous errors happen immediately and can be caught with try...catch blocks.

2. Step 2: Pass error to Express error handler

   Inside catch, call next(error) to forward the error to Express's error middleware.

3. Final Answer:

   Use a try...catch block and call next(error) inside catch -> Option D

4. Quick Check:

   try...catch + next(error) = A [OK]

Hint: Catch sync errors with try...catch and call next(error) [OK]

Common Mistakes:

• Not using try...catch for synchronous errors
• Calling next() without an error object
• Using async/await without try...catch for sync errors

2. Which of the following is the correct syntax to pass a synchronous error to the next middleware in Express?

easy

A. next(new Error('Something went wrong'))

B. throw new Error('Something went wrong')

C. res.send(new Error('Something went wrong'))

D. console.error('Something went wrong')

Solution

1. Step 1: Identify how to forward errors in Express

   Express expects errors to be passed to next() as an argument to trigger error middleware.

2. Step 2: Check syntax correctness

   Calling next(new Error('...')) correctly passes the error object to Express.

3. Final Answer:

   next(new Error('Something went wrong')) -> Option A

4. Quick Check:

   next(error) syntax = B [OK]

Hint: Use next(error) to forward errors in Express [OK]

Common Mistakes:

• Throwing error without try...catch in sync code
• Sending error object in response instead of next()
• Logging error without forwarding it

3. What will be the output when this Express route is called?

app.get('/test', (req, res, next) => {
  try {
    throw new Error('Fail');
  } catch (err) {
    next(err);
  }
});

app.use((err, req, res, next) => {
  res.status(500).send(err.message);
});

medium

A. The server crashes with uncaught exception

B. The client receives a 200 status with empty body

C. The client receives a 500 status with message 'Fail'

D. The client receives a 404 Not Found error

Solution

1. Step 1: Analyze error throwing and catching

   The route throws an error inside try, caught immediately, then passed to next(err).

2. Step 2: Understand error middleware response

   The error middleware sends a 500 status with the error message 'Fail' to the client.

3. Final Answer:

   The client receives a 500 status with message 'Fail' -> Option C

4. Quick Check:

   next(err) triggers error middleware = A [OK]

Hint: Errors passed to next() trigger error middleware response [OK]

Common Mistakes:

• Assuming error crashes server without middleware
• Expecting 200 status despite error
• Confusing error middleware with normal middleware

4. Identify the error in this Express route for synchronous error handling:

app.get('/error', (req, res, next) => {
  throw new Error('Oops');
  next();
});

medium

A. No error, code works fine

B. Error is thrown but not caught or passed to next()

C. Error middleware is missing

D. next() is called after throw, which is unreachable

Solution

1. Step 1: Check error throwing without try...catch

   The error is thrown synchronously but not caught, so Express cannot handle it properly.

2. Step 2: Understand consequence of uncaught error

   Without try...catch and calling next(err), the server may crash or not handle error gracefully.

3. Final Answer:

   Error is thrown but not caught or passed to next() -> Option B

4. Quick Check:

   Throw without catch + next(err) = D [OK]

Hint: Always catch sync errors and call next(error) [OK]

Common Mistakes:

• Throwing error without try...catch
• Calling next() after throw (unreachable code)
• Assuming Express auto-catches thrown errors

5. You want to handle synchronous errors in multiple routes without repeating try...catch blocks. Which Express feature helps you achieve this cleanly?

hard

A. Create a wrapper function that catches errors and calls next(err)

B. Use setTimeout to delay error throwing

C. Call res.end() immediately to avoid errors

D. Use global variables to store errors

Solution

1. Step 1: Recognize repetitive try...catch problem

   Wrapping every route in try...catch is repetitive and cluttered.

2. Step 2: Use a wrapper function to centralize error catching

   A wrapper function can run the route handler and catch sync errors, forwarding them with next(err).

3. Final Answer:

   Create a wrapper function that catches errors and calls next(err) -> Option A

4. Quick Check:

   Wrapper function for sync error handling = C [OK]

Hint: Use wrapper functions to catch and forward sync errors [OK]

Common Mistakes:

• Trying to delay errors with setTimeout
• Ignoring errors by ending response early
• Using global variables for error handling