Error Handling and Logging

Error handling refers to the programming practice of anticipating and coding for error conditions that may arise when your program runs. In this chapter, we're going to learn how to handle errors in Node.js.

Error Handling

We cannot assure our code runs well. Because of it, we should think about how to handle errors. To do so, we can use try..catch syntax. Here is the error handling format in Node.js:

Let’s write this code.

You can see the n/b operation returns an Infinity value. In this situation, we can throw our error so it will be caught.

If you run this code, you will get the program output shown in Figure 47.

Figure 47: Catching an error in Node.js

Logging

In the previous section, we used console.log() to print an error message in the console. Imagine there are many messages in the console and we want to identify which error message is. The simple solution is to apply font color to the message to identify an error occurring.

Alternatively we can use log4js-node. To install this module, write this script:

For Windows platform, you should run it under administrator level. You can do it using RunAs in console

If you don’t install it globally, you can execute as follows:

If it is installed successfully, you will get a response output as shown in Figure 48.

Figure 48: Installing log4js-node

How do we use this module? Let’s write the sample code:

Save it into a file called logger-demo.js. Run the file:

You will see the program output as shown in Figure 49. You can see log4js-node display color text is based on log type.

You could change the display text (default) by application category. Just write the category on getLogger(), for instance, myapplication.

Run it again and you will get a new output response, shown in Figure 50.

Figure 49: Running application with log4js-node

Figure 50: Changing default value with own category name

By default, log4js-node writes message on console. If you want to write all messages in a file, you can configure a logging file. To get started:

1. Activate file logging by calling loadAppender()
2. Configure the file logging configuration and category name.

For instance, we store the log file on c:\temp\myapplication.log with the category name myapplication. Here is a code illustration:

Run this code. Then, you will get a log file, myapplication.log. If you open it, you will see the file content as follows:

For further information about log4js-node, visit https://github.com/nomiddlename/log4js-node.