[Bug]: PROCESS.ENV.ASSET_PREFIX is replaced with "auto"

[giancarlosisasi]

Version

System:
    OS: macOS 15.0.1
    CPU: (8) arm64 Apple M1
    Memory: 79.30 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Browsers:
    Brave Browser: 132.1.74.50
    Chrome: 133.0.6943.99
    Safari: 18.0.1

Details

In my library, we sanitize and export a set of env variables that can be used in our web apps:

export const squared = (n: number): number => n * n;

export const envVars = {
  ASSET_PREFIX: process.env.ASSET_PREFIX,
}

But when compiling it with rslib, the process.env.ASSET_PREFIX is replaces with the "auto" value 🤔

Code after compilation:

const squared = (n)=>n * n;
const envVars = {
    ASSET_PREFIX: "auto"
};
export { envVars, squared };

I have not configured anything about that in my rslib.config.ts file so I would expect to have this without a value and instead keep the original code like:

// compiled code
const squared = (n)=>n * n;
const envVars = {
    ASSET_PREFIX: process.env.ASSET_PREFIX_, // <-- this should keep as the original code
};
export { envVars, squared };

Reproduce link

https://github.com/giancarlosisasi/rslib-bug-env-var

Reproduce Steps

1. git clone https://github.com/giancarlosisasi/rslib-bug-env-var
2. pnpm install && pnpm build
3. Check the file dist/index.js, there you can see the "auto" value in the envVars.ASSET_PREFIX even when that variable was not configured in the rslib.config.ts

[Timeless0911]

This is related to asset module support in Rslib that when format is set to cjs or esm, Rslib defaults to setting output.assetPrefix to "auto" since Rslib treats the output as an mid-level artifact that will be consumed by other build tools again.

Even if Rslib does not automatically set it to auto, the default value of process.env.ASSET_PREFIX is "/" and Rsbuild will automatically remove the trailing slash from assetPrefix to make string concatenation easier like below.

const envVars = {
    ASSET_PREFIX: ""
};

see

• output.assetPrefix
• Environment variables - process.env.ASSET_PREFIX

If you want to keep the original code, you should use source.define:

source: {
  define: {
    'process.env.ASSET_PREFIX': 'process.env.ASSET_PREFIX',
  },
}

[giancarlosisasi]

@Timeless0911 oh, that makes sense. Maybe it would be interesting if that behaviour can change in the future because, maybe, in a context if a library assetPrefix is not commonly used 🤔

Anyways, thanks for the source.define suggest! that works! 👍🏼 😄

[Timeless0911]

It is because assetPrefix is ​​not common in library development, Rslib is set to "auto" by default, which is different from Rsbuild.

Since Rslib use the webpack arch to process static assets, in order to ensure the correctness of the path, assetPrefix needs to be set to "auto", otherwise additional publicPath runtime code will be generated, see #570 #684