Understanding Environment Variables in Next.js
Environment variables in Next.js are crucial for managing configuration and keeping sensitive data, like API keys and database passwords, secure and out of your codebase. In Next.js, environment variables are typically defined in a .env file, which is placed at the root of the project. These variables can then be accessed in your application using process .env.YOUR_VARIABLE_NAME. Next.js also supports different sets of environment variables for development and production environments, enabling you to manage your application's configuration seamlessly across multiple stages.
To ensure these variables are only exposed where necessary, Next.js differentiates between server-side and client-side variables, with the latter requiring a specific prefix (NEXTPUBLIC) to make them accessible in the browser. This system not only enhances security but also aids in the maintainable and scalable configuration of Next.js applications.
Common Reasons Why Environment Variables Are Undefined in Next.js
Incorrect .env File Placement: Next.js expects environment variable files to be located at the root of your project. If the .env file is placed in a subdirectory or incorrectly named, the variables won’t be loaded.
Naming Mistakes:
Environment variables must be named correctly in your .env files. Simple typos or using reserved JavaScript keywords as names can lead to variables that are either undefined or not behaving as expected.
Improper Access in Code:
Next.js requires that environment variables to be used in the browser have names prefixed with NEXTPUBLIC. Without this prefix, variables are only available on the server-side, leading to them being undefined in the client-side code.
Not Restarting the Server:
After adding new variables to your .env files, you need to restart your Next.js server. The environment variables are loaded when the server starts, so changes made while the server is running won't be recognized until a restart.
Variable Overwrites:
If environment variables are set both in .env files and externally (like in a CI/CD pipeline), there might be conflicts or overwrites that cause some variables to be undefined due to unexpected values or overwrites.
Scope of Environment Variables:
Developers sometimes forget that environment variables are not reactive and won't update within a running session if their values change externally. They need to be reloaded by restarting the server or application.
Build-Time vs. Runtime Variables:
Next.js statically optimizes pages at build time. If environment variables are expected to be available at runtime, but are only included at build time, they will not appear as expected when the application is running.