Feature Flags

Feature Flags

Feature flags are used to enable or disable specific features provided. The features option in the configuration allows you to control the availability of these features.

Syntax for Specifying Flags

  • true: Enables the feature for all files.
  • false: Disables the feature for all files.
  • "glob": Enables the feature only for files that match the specified glob pattern.
  • ["glob1", "glob2"]: Enables the feature for files matching any of the specified glob patterns.
  • ["glob1", "!glob2"]: Enables the feature for files matching glob1 but excludes files that match glob2.

dangerousCodeRemover Feature

The dangerousCodeRemover is a flag that is enabled by default. It is designed to enhance the static evaluation of values that are interpolated in styles and to optimize the processing of styled-wrapped components during the build stage. This optimization is crucial for maintaining a stable and fast build process. It is important to note that the dangerousCodeRemover does not impact the runtime code; it solely focuses on the code used during the build.

How It Works

During the build process, WyW-in-JS statically analyzes the CSS-in-JS codebase and evaluates the styles and values that are being interpolated. The dangerousCodeRemover steps in at this stage to remove potentially unsafe code, which includes code that might interact with browser-specific APIs, make HTTP requests, or perform other runtime-specific operations. By removing such code, the evaluation becomes more reliable, predictable, and efficient.


Enabling the dangerousCodeRemover feature provides several benefits:

  1. Stability: The removal of potentially unsafe code ensures that the build process remains stable. It minimizes the chances of encountering build-time errors caused by unsupported browser APIs or non-static operations.

  2. Performance: Removing unnecessary code results in faster build times. The build tool can efficiently process and evaluate the styles and components without unnecessary overhead, leading to quicker development cycles.

Fine-Tuning the Removal

While the dangerousCodeRemover is highly effective at optimizing the build process, there may be cases where it becomes overly aggressive and removes code that is actually required for your specific use case. In such situations, you have the flexibility to fine-tune the behavior of the remover.

By leveraging the features option in the configuration, you can selectively disable the dangerousCodeRemover for specific files. This allows you to preserve valuable code that may not be safely evaluated during the build process.


Suppose you have a file named specialComponent.js that contains code that should not be deleted. By adding the following entry to your features configuration:

  features: {
    dangerousCodeRemover: ["**/*", "!**/specialComponent.js"],

You are instructing WyW-in-JS to exclude the specialComponent.js file from the removal process. As a result, any code within this file that would have been removed by the dangerousCodeRemover will be retained in the build output.

globalCache Feature

The globalCache is enabled by default. WyW-in-JS uses two levels of caching to improve the performance of the build process. The first level is used to cache transformation and evaluation results for each transform call, usually a single call of Webpack's loader or Vite's transform hook. The second level is used to cache the results of the entire build process. The globalCache feature controls the second level of caching. Turning off this feature will result in a slower build process but decreased memory usage.

happyDOM Feature

The happyDOM is enabled by default. This feature enables usage of https://github.com/capricorn86/happy-dom (opens in a new tab) to emulate a browser environment during the build process. Typically, the dangerousCodeRemover feature should remove all browser-related code. However, some libraries may still contain browser-specific code that cannot be statically evaluated. In such cases, the happyDOM feature can be used to emulate a browser environment during the build process. This allows WyW-in-JS to evaluate the code without encountering errors caused by missing browser APIs.

softErrors Feature

The softErrors is disabled by default. It is designed to provide a more lenient evaluation of styles and values that are interpolated in styles. This flag is useful for debugging and prevents the build from failing even if some files cannot be processed with WyW-in-JS.

'useBabelConfigs' Feature

The useBabelConfigs feature is enabled by default. If it is enabled, WyW-in-JS will try to resolve the .babelrc file for each processed file. Otherwise, it will use the default Babel configuration from babelOptions in the configuration.

Please note that the default value of useBabelConfigs will be changed to false in the next major release.