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The recommended way to conditionally detect when code is being executed with bun is to check for the existence of the Bun global. This is similar to how you’d check for the existence of the window variable to detect when code is being executed in a browser. 
if (typeof Bun !== 'undefined') {
	// this code will only run when the file is run with Bun
}
In TypeScript environments, the previous approach will result in a type error unless @types/bun is installed. To avoid this, you can check process.versions instead. 
if (process.versions.bun) {
	// this code will only run when the file is run with Bun
}