bun:bundle

• function feature(

  flag: string

  ): boolean;

  Check if a feature flag is enabled at compile time.

  This function is replaced with a boolean literal (true or false) at bundle time, enabling dead-code elimination. The bundler will remove unreachable branches.

  @param flag

  The name of the feature flag to check

  @returns

  true if the flag was passed via --feature=FLAG, false otherwise

  import { feature } from "bun:bundle";
  
  // With --feature=DEBUG, this becomes: if (true) { ... }
  // Without --feature=DEBUG, this becomes: if (false) { ... }
  if (feature("DEBUG")) {
    console.log("Debug mode enabled");
  }
  

Type definitions

• interface Registry

  Registry for type-safe feature flags.

  Augment this interface to get autocomplete and type checking for your feature flags:

  // env.d.ts
  declare module "bun:bundle" {
    interface Registry {
      features: "DEBUG" | "PREMIUM" | "BETA";
    }
  }
  

  Now feature() only accepts "DEBUG", "PREMIUM", or "BETA":

  feature("DEBUG");    // OK
  feature("TYPO");     // Type error