Error handling in express.js

Introduction

In this article, I used the code in the following repository:
https://github.com/rezahazegh/demo-express-error-handling

To demonstrate how express.js behaves regarding error handling by using some pretty straightforward examples. Then, I provided a solution to have cleaner syntax to handle errors.

Hint: express.js behavior is different for version 5 and above.

Trail and Error

The project includes some routes which show express behavior under different conditions:

1. /test

   • We use it to check either app crashed or still working

2. /sync

   • If you call this route it throws an error from within a sync route handler.
   • if we call test the app would work. So, express catches it automatically and, we don't need any extra action for sync handlers.

3. /async-not-handled

   • If you call this route it throws an error from within an async route handler which is not handled.
   • If you call test the app won't work and, you will see it already crashed.
   • So, we need to somehow handle errors for async handlers. we show how to do that on the next ones.

4. /async-handled-regular

   • If you call this route it throws an error from within an async route handler which is handled using try/catch.
   • The point is you should pass error to next function.
   • Now, if we call test the app would work.

Magic solution

The following solution helps you to get rid of try/catch in your handlers.

Functions in javascript are first class citizens:

• you can pass a function as a parameter to another one.
• Also, you can return a function as a return value.

We are going to use these features to design our solution.

handler function helps us to extract try/catch from our handlers. It takes our handler as a parameter and wrap it in a try/catch in a new function. Then return the new function.

1. /async-handled-using-handler
   • If you call this route it throws an error from within an async route handler which is handled using handler function.
   • Now, if we call test the app would work.