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Check process.versions.bun to detect whether code is running in Bun. This works in both JavaScript and TypeScript without extra type definitions. 
if (process.versions.bun) {
  // this code will only run when the file is run with Bun
}
Alternatively, check for the Bun global, the same way you’d check for window to detect a browser.
In TypeScript, this is a type error unless @types/bun is installed. Install it with bun add -d @types/bun.
if (typeof Bun !== "undefined") {
  // this code will only run when the file is run with Bun
}