Function run_npm_install 

fn run_npm_install(
    dir: &Path,
    package: &str,
    background: bool,
) -> Result<NpmRun>

Run npm install --prefix <dir> <package> to completion. package is one or more space-separated name@version specs.

Output is collected after exit (.output()): pipe-based streaming (async IOCP and sync blocking alike) receives nothing from cmd.exe /C batch children on Windows until the process exits.