Understanding Variable Scope in REST APIs: How to Avoid Shared State Issues in Node.js

Discover why two users accessing the same variable in your REST API can lead to unexpected results and learn how scoping helps maintain data integrity.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why can two user accessing the same variable within my REST API

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---

As developers delve into building applications using REST APIs, they often encounter various challenges. One common issue arises when multiple users access the same variable concurrently, leading to unexpected results. This problem can be particularly evident in scenarios where functions are called frequently, such as in a music streaming service utilizing an API like Spotify.

The Issue Explained

Let's consider a situation where you have built a REST API endpoint /get-spotify-data that retrieves the currently playing track for users. In your code, you're utilizing a variable called artists_names to store the names of artists associated with a track. As users interact with your application, they notice that:

When two users make calls to the /get-spotify-data endpoint at the same time, they receive alternating responses containing artists' names from each other's songs.

Other data, such as track names and album covers, is sent correctly.

This unexpected behavior hints at a problem with how you are handling the variable artists_names within your API.

Why Does This Happen?

Variables that are declared without let, const, or var are treated as global variables, leading to shared state, which causes conflicts when multiple requests overwrite each other’s data.

When the same variable is accessed by multiple users simultaneously, the result can be unpredictable since one user's data modifies the other’s.

Solution: Proper Variable Scoping

To resolve this issue, you need to ensure that your variables are scoped correctly within your API. Here are some strategies to achieve that:

1. Use let or const

Use let or const to declare your variables within the function scope. This keeps them separate for each function call.

[[See Video to Reveal this Text or Code Snippet]]

2. Limit Global Variables

Be mindful of where you declare your variables. Avoid using global variables unless absolutely necessary. If you need a variable to persist data across requests, consider using proper session management or caching.

3. Test Concurrency

Run tests simulating concurrent requests to ensure that your changes effectively isolate user data. Tools like Apache JMeter or Postman can help simulate multiple users accessing your API simultaneously.

Conclusion

Now, whenever you're building an API, remember the importance of scoping and how it impacts your application. Happy coding!