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How to write Custom Error Handler Middleware in Express.js using JavaScript 👩‍💻

Bentil Shadrack on August 20, 2022

What is Error Handler Error handler is responsible of identifying and handling runtime issues. Express.js comes pre-configured with a b...

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bbonsaye • Jan 3 '23

Hey, thank you for this article. Do you mind sharing how your routes file is structured? The way mine is structured is that all the routes are on the router object and I export that route object and within app.js I have

app.use(authRoutes)

I like how yours is explicit, I'm not sure how I can import each route by name the way you have it setup.
Thanks in advance.

Bentil Shadrack • Jan 3 '23

Project Structure

Routes Structure:

This is how each route looks like

For easy export and import, I have created the index.route.js to export all routes from a single file

So I can easily import the routes like this

Instead of importing them from their separate files.

This is a typical backend repo of mine. You can check it out
github.com/qbentil/funchat-backend

bbonsaye • Jan 3 '23 • Edited

Thank you for sharing that, I'm going to separate my routes like that too. If I may, I seen a video of someone who had a ".env" file in the root directory and a config folder located in "/src/config" and within that config.js file, they basically copied the variables in ".env" and pasted them in "config.js" and exported them to be used in their code.

Right now, I only have the ".env" file and access the variables located within it using the "process.env" object within my code, example: "process.env.HOST"

I like the idea of accessing the environment variables like so; "config.HOSTNAME" but, I find it redundant and time consuming to always update "/configs/config.js" every time we add a new variable to ".env"

Is that what you are doing or have you found a way where every time you add/remove a new variable in ".env" it automatically adds/removes the variable in "/configs/config.js"?

by the way, I just checked out your GitHub Portfolio briefly and noticed a minor typo. In your "ABOUT ME" you have "(Coud Storage)" I think you meant, "(Cloud Storage" -- tryna help hope that's not annoying to point out.

Bentil Shadrack • Jan 3 '23

I also use the same approach you are using now.
I access the env variables through the process.env.NAME

Thank you very much for the feedback on my readme. I really Appreciate.🥰🎉
I will check it out asap

Miguel Z. Nunez • Feb 4 '23

Can you show me how I would go about implementing this error handler with try catch in the following GET route? The route makes a query to a database table named images and sends the data over to the frontend. If there is some kind of error in accessing the data, I'm not sure how to go about grabbing the database error. I am using "throw new Error" but that only stops the program on its tracks. I want to be able to get the error in JSON and send the error to the user if possible.

app.get("/", (req, res) => {
dbConnection.query("SELECT * FROM images WHERE user_id = ?", [1], (err, result) => {
if(!err) res.render("index", {images:result})
else throw new Error(err)
})
})

Bentil Shadrack • Mar 15 '23

Hello Miguel, You can put it this way if you have your errorhandler middleware in place

app.get("/", (req, res, next) => {
    try {
        dbConnection.query("SELECT * FROM images WHERE user_id = ?", [1], (err, result) => {
            res.render("index", { images: result })
        })
    } catch (error) {
        next(error)
    }
})

I hope this help☺

javad-sh • Dec 8 '22

Even if we don't use a try and catch and even if we don't use next(), the errors in the route are accessible in error handler middleware.
What is the reason for using the try and catch?
I would also be grateful if you could tell me how to customize the errors of each root in itself.I mean how to set for example error message and or name in every route.

Bentil Shadrack • Dec 8 '22

Without the next() The errors will not be handled by your error Handler middleware. It will be handled by Express framework’s default error handler

Errors that are created by asynchronous functions that are called from route handlers, need to be handled differently. The error from asynchronous functions are not handled by the default error handler in Express which can result in crashing the entire App. Putting then in a try catch block will help you handle those errors separately using you custom error handler