Error Handling in ExpressJS

Introduction

Error handling is an important aspect of any web application development. In an Express.js application, error handling can be implemented using middleware functions that are specifically designed to catch and handle errors. These middleware functions are executed in the order in which they are defined in the code, and they can be used to handle different types of errors, such as 404 errors, database errors, and authentication errors.

One of the most common error handling middleware functions in Express.js is the error middleware function, which is used to handle all errors that occur in the application. This function is typically defined at the end of the middleware chain, and it takes four parameters, namely err, req, res, and next. The err parameter represents the error object, while the req and res parameters represent the request and response objects, respectively. The next parameter is used to call the next middleware function in the chain.

In addition to the error middleware function, Express.js provides other middleware functions that can be used for error handling. For example, the next middleware function can be used to pass control to the next middleware function in the chain if an error occurs. The catch middleware function can also be used to catch errors that occur during asynchronous operations, such as database queries and file system operations.

Overall, error handling is a critical component of any Express.js application, and developers should take the time to understand the various middleware functions available for handling errors. By implementing effective error handling, developers can ensure that their applications are more robust and reliable and that they can provide a better user experience for their users.

Sources of errors

There are various sources of errors that can occur in ExpressJs. Some of the most common sources of errors include:

1. Input errors: Whenever we request a path in our browser or API testing platform, and if there is no route handler created for it, it will throw an error in our express application. For instance, let us say we have a route in our index.js file / which is a get request. This route will land us on the home page of our application. But if in the browser I mistakenly make a get request with an endpoint as /home, it will give me an error, as there is no route handler defined for this /home path.
2. Logical errors: Another error might occur when there is something goes wrong in the route handler or some other part of the code. For instance, in the above route / let us say we mimic an error by throwing an error to break the app. Let the code of the route / in the index.js file will be as follows:

   app.get(‘/’, (req, res, next) => {
    //Mimic an error by throwing an error to break the app!
    throw new Error(‘There is some issue occurred);
    res.send(‘Welcome Home!’)
   })
   

   Then if you run your application, and make a get request with an endpoint as /, we will see there will be an error on that page stating There is some issue occurred.
3. Syntax errors: These are errors that occur when the code does not follow the correct syntax of the programming language. Examples include missing or misplaced brackets, semicolons, and parentheses.
4. External errors: These are errors that occur due to external factors such as network connectivity issues, database connection failures, or third-party API errors.

To effectively handle errors in an Express.js application, it is important to identify the source of the error and implement the appropriate error-handling mechanism. By understanding the sources of errors, developers can write more robust and reliable code, and ensure that their applications provide a better user experience.

Error Handling while Calling Promise-based Methods

To maintain the stability and functionality of your ExpressJS application, it's crucial to handle errors correctly when calling Promise-based methods. The following are some helpful tips on how to effectively manage errors in Promise-based methods in ExpressJS:

Use tries/catch blocks: Wrap your Promise-based method calls in try/catch blocks to catch any errors that may occur during execution. This will allow you to handle the error in a more controlled manner and provide a meaningful response to the client.

Use Promise.catch(): Promise-based methods return Promises which can be chained with a .catch() method to handle any errors that occur during execution. This method allows you to handle errors in a more declarative way and can make your code more readable.

Use next(err): In an ExpressJS application that uses middleware functions, you can pass an error to the next function in the chain by utilizing the next() method. By doing so, you can handle errors in a centralized manner which promotes modularity and better organization of your code.

Use a global error handler: Creating a middleware function for global error handling is another useful technique. It enables you to manage any unanticipated errors that may happen during the execution process. This will result in a more centralized approach to error handling and can contribute to enhancing the maintainability of your code.

Use error handling libraries: There are several error handling libraries available for ExpressJS, such as Express-async-errors and Boom. These libraries provide additional functionality for handling errors and can make your code more robust.

Remember to always handle errors properly in your ExpressJS application to ensure that it remains stable and functional.

Handling Routing Errors by Ordering Routes

In ExpressJS, routes are matched in the order they are defined. This means that the first route that matches the request will be used to handle the request.

If there are multiple routes that match the request, the first matching route will be used. Therefore, it's important to order your routes carefully so that the most specific routes come before the more general ones.

For example, if you have a route that matches /users/:userId and another route that matches /users/new, you should define the /users/:userId route before the /users/new route. Otherwise, the /users/new route will match requests for /users/1234, resulting in a routing error with the message Cannot GET /1234.

Example:

here's an example to illustrate the importance of ordering routes in ExpressJS

Suppose we have the following routes defined in our ExpressJS application:

In this case, if a user requests to /users/new, the first route defined (/users/new) will match the request, and the callback function associated with this route will be called. The same goes for requests to /users/1234. The second route (/users/:userId) will match the request and the callback function associated with this route will be called.

However, this could lead to a routing error if we swap the order of the routes like this:

Now, if a user requests to /users/new, the first route defined (/users/:userId) will match the request, resulting in a routing error. Similarly, if a user requests /users/1234, the second route defined (/users/new) will match the request, resulting in a routing error.

So, it's important to order the routes in a way that ensures the most specific route comes before the more general one to avoid routing errors.

In general, it's best to define your routes in a way that avoids overlapping routes as much as possible. This will help ensure that requests are handled correctly and that routing errors are minimized.

Centralized Error Handling: How to exploit the order of the router

In ExpressJS, the order in which middleware and routes are defined matters. You can take advantage of this order to implement centralized error handling.

First, you can define a middleware function that will catch any errors thrown by subsequent middleware or route handlers. This middleware function should have four parameters: err, req, res, and next. err is the error object that was thrown, req is the request object, res is the response object, and next is the next middleware function in the chain.

Next, you should define your routes and middleware in the order in which they should be executed. Any routes or middleware defined after the error handler middleware will not be executed if an error is thrown.

Explanation: In the example above, the errorHandler middleware is defined after all other middleware and routes. If an error is thrown by any of the previous middleware or routes, the error handler middleware will catch it and respond with a 500 error.

Note that you can also define multiple error handling middleware functions in case you want to handle different types of errors differently. In that case, the order of the error handling middleware functions matters as well.

Calling the Error Handling Middleware Function

Middleware functions are a powerful feature of web frameworks that allow developers to add additional functionality to their applications. They can be used for various purposes, including authentication, logging, and error handling. In this section, we will focus specifically on calling the error handling middleware function.

Middleware functions for handling errors play a crucial role in Node.js and Express.js applications. Their purpose is to manage errors that may arise during the processing of a request-response cycle and provide relevant error messages to clients. In this section, I will provide a comprehensive explanation of the process involved in calling an error handling middleware function in Express.js, and provide examples to illustrate each step.

To invoke an error handling middleware function in Express.js, you need to first define the function and then register it in your application. The following are the steps that you need to follow:

Step 1: Define the error handling middleware function

The error handling middleware function should accept four parameters, namely error, request, response, and next. The error parameter contains the error object, while the request and response parameters represent the request and response objects, respectively. The next parameter is a function that allows for control to be passed to the next middleware function in the stack.

Example:

Here is an example of an error handling middleware function:

Explanation:

The code block above shows an example of an error handling middleware function in Express.js. The purpose of this function is to handle errors that may occur during the processing of a request-response cycle and return an appropriate error message to the client.

The function logs the error stack to the console using console.error(err.stack), which is helpful for debugging purposes. Then it sends a response to the client using res.status(500).send('Something broke!'). This sets the status code to 500, which indicates that an internal server error has occurred, and sends the message Something broke! to the client.

Step 2: The second step in calling an error handling middleware function in Express.js involves registering the function by adding it to the middleware stack using the app.use() method. It's important to note that the error handling middleware function should be the last middleware function added to the stack.

Here's an example that illustrates how to register the error handling middleware function:

Explanation:

In the example above, the errorHandler function is defined as the error handling middleware function. Then, it's registered using app.use(errorHandler). This ensures that any errors that occur during the request-response cycle will be handled by this function.

Note that app.use() can also take a path parameter to specify which routes the middleware function should be applied to. If no path parameter is provided, the middleware function will be applied to all routes.

Step 3: Trigger the error handling middleware function

The error handling middleware function is triggered when an error occurs in any of the middleware functions in the stack. To trigger the error handling middleware function, you can throw an error object or pass an error object to the next() function.

Here is an example of how to trigger the error handling middleware function:

Explanation:

In the above example, the error handling middleware function is triggered when the id parameter is equal to 1. The next() function is used to pass control to the error handling middleware function and pass an error object to it.

That's it! You now know how to call the error handling middleware function in Express.js. Remember to add the error handling middleware function as the last middleware function in the stack and trigger it by throwing an error object or passing an error object to the next() function.

Adding Multiple Middleware Functions for Error Handling

In general, error handling middleware functions are used to catch and handle any errors that occur during the processing of a request. They can be used to log errors, send error messages to the client, or perform other actions as needed.

When working with middleware functions for error handling, it is common to use a chain of multiple functions. Each function in the chain is responsible for handling a specific type of error. If an error occurs that is not handled by any of the middleware functions, it will be caught by the default error handler provided by the framework.

Sometimes, it is necessary to use multiple error handling middleware functions to handle different types of errors differently. In such cases, Express JS allows developers to chain multiple middleware functions together to form a pipeline that handles errors.

Each middleware function in the pipeline is responsible for catching a specific type of error and handling it appropriately. If an error occurs that is not handled by any of the middleware functions, it will be caught by the default error handler provided by Express JS.

Here is an example of using multiple middleware functions for error handling in an Express JS application:

Explanation:

In this example, we have four middleware functions in the error handling pipeline, each handling a specific type of error:

handleInvalidJson handles errors that occur when the request body contains invalid JSON. handleUnauthorized handles errors that occur when the user is not authorized to access a resource. handleNotFound handles errors that occur when a requested resource is not found. handleAllOtherErrors handles all other errors that are not caught by the previous middleware functions. These middleware functions are added to the pipeline with the help of the app.use() method. The app.use() method adds these middleware functions to the middleware stack. The sequence in which they are appended to the stack is significant, as each middleware function runs in the order of its addition.

In addition to the error handling middleware functions, we also define a simple route that responds with Hello World! when the root URL is requested.

If an error occurs during the request processing, it will be caught by one of the middleware functions in the pipeline. If an error is not caught by any of the middleware functions, it will be caught by the default error handler provided by Express JS.

Using multiple middleware functions for error handling in an Express JS application is a powerful technique that allows developers to handle different types of errors differently. It is essential to use this technique to build robust and reliable applications.