rollup-plugin-inject is a Rollup plugin providing a simple way to inject variables or modules into your JavaScript bundles. It replaces identifiers in your code with specified values or requires. This is particularly useful for polyfilling environment-specific variables like process.env.NODE_ENV or providing mock implementations during testing. Configuration is straightforward, allowing you to easily define what should be injected and where. Offers a concise solution for dependency injection and conditional compilation within Rollup.