When Val Town's code registry (esm.town) gets a request from a browser (which it can detect via the User Agent header), it transpiles that val's code to web-standard JavaScript be able to run in the browser.

It doesn't do as much as other transpilers (such as esm.sh, such as rewriting npm: imports, etc). We may add that capability in the future. For now, if you want your npm imports to run in the browser, use https://esm.sh/package instead of npm:package.

The below script demonstrates this transiplation behavior by fetching its own source code (import.meta.url) with the user agent of a browser. You can uncoment the line setting the browser agent if you want to see the difference in the output. Or you could just load this val's module URL in your browser to see the untranspiled TS.

As of July 23, 2024, this is the code that determines when esm.town transpiles or not: