[Feature request] Make public url parameter available inside the app

[caizixian]

I'd like to revive #5121, and provide more context.

🙋 feature request

Make public url parameter available inside the app.

🤔 Expected Behavior

Expose the argument to parcel --public-url to the user code, such as via process.env.PUBLIC_URL.

😯 Current Behavior

There's apparently no way to retrieve the argument to parcel --public-url programmatically in the user code.

💁 Possible Solution

A workaround is to explicitly specify the public url PUBLIC_URL=<public url> parcel build --public-url <public url>.

🔦 Context

For example, when deploying single page apps built using React Router under a subfolder, it's cleaner to pass the base url to BrowserRouter programmatically. This is useful when, for example, deploying SPA using GitHub pages, when the site would be available under <username>.github.io/<repo name>

💻 Examples

See the above context. webpack seems to have some support https://webpack.js.org/guides/public-path/

[devongovett]

This will be supported via import.meta.publicUrl in the next release. It was already implemented as part of #10043.