Function build_my_react_js_fallible 

pub fn build_my_react_js_fallible(
    path: &str,
    outer_env: &str,
    silent: bool,
) -> Result<(), String>

This performs the following:

Check for a build/index.html file, an indication that a React build has previously succeeded in the indicated crate subdirectory

Check for NPM and connection to servers using npm ping If first run try NPM install to fetch deps Check for NPM and connection to servers, possibly again Attempt to build using npm run build

After the build has succeeded once, subsequent runs will instruct your cargo to only run build.rs on updates.