When would you use global variables in Node.js? Are they always bad?

When considering the use of global variables in Node.js, it's important to weigh both their advantages and disadvantages to determine when their use might be appropriate or necessary.

Advantages of Using Global Variables

1. Shared Access Across Modules: Global variables can be useful for storing values that need to be shared across different parts of an application without the need to pass them explicitly to each module or function[2].
2. Configuration or Constant Values: They are often used to store configuration settings or constants that are needed throughout the application, such as API keys, database configuration settings, or application-wide settings[5].

Disadvantages of Using Global Variables

1. Memory Consumption: Global variables persist in memory for the duration of the application's runtime, which can lead to increased memory usage and potential memory leaks if not managed carefully[1].
2. Maintainability Issues: They can lead to code that is harder to understand and maintain. Since any part of the application can change a global variable, it can become difficult to track where values are being set or modified[6][7].
3. Testing Challenges: Testing can become more complex because changes to global variables in one part of the application can affect other parts that are being tested, leading to potential side effects and making it harder to isolate tests[9].
4. Concurrency Issues: In multi-threaded environments, global variables can lead to issues such as race conditions, although this is less of a concern in Node.js given its single-threaded nature. However, in clustered Node.js environments, this could still pose a problem[1].

When to Use Global Variables

Despite these disadvantages, there are scenarios where global variables can be the right choice:

1. Storing Application-Wide Constants: If you have constants that don’t change and are used across many parts of your application, such as error codes or configuration flags, global variables can be an efficient way to access these values[5].
2. Singleton Patterns: In cases where a single instance of a particular object is supposed to exist throughout the application, such as a configurat...