packages/react-dev-utils/WebpackDevServerUtils.js
function onProxyError(proxy) {
  return (err, req, res) => {
    const host = req.headers && req.headers.host;
    console.log(
      chalk.red('Proxy error:') +
        ' Could not proxy request ' +
        chalk.cyan(req.url) +
        ' from ' +
        chalk.cyan(host) +
        ' to ' +
        chalk.cyan(proxy) +
        '.'
    );
    console.log(
      'See https://nodejs.org/api/errors.html#errors_common_system_errors for more information (' +
        chalk.cyan(err.code) +
        ').'
    );
    console.log();

    // And immediately send the proper error response to the client.
    // Otherwise, the request will eventually timeout with ERR_EMPTY_RESPONSE on the client side.
    if (res.writeHead && !res.headersSent) {
      res.writeHead(500);
    }
    res.end(
      'Proxy error: Could not proxy request ' +
        req.url +
        ' from ' +
        host +
        ' to ' +
        proxy +
        ' (' +
        err.code +
        ').'
    );
  };
}
function onProxyError(proxy) {
  return (err, req, res) => {
    const host = req.headers && req.headers.host;
    console.log(
      chalk.red('Proxy error:') +
        ' Could not proxy request ' +
        chalk.cyan(req.url) +
        ' from ' +
        chalk.cyan(host) +
        ' to ' +
        chalk.cyan(proxy) +
        '.'
    );
    console.log(
      'See https://nodejs.org/api/errors.html#errors_common_system_errors for more information (' +
        chalk.cyan(err.code) +
        ').'
    );
    console.log();

    // And immediately send the proper error response to the client.
    // Otherwise, the request will eventually timeout with ERR_EMPTY_RESPONSE on the client side.
    if (res.writeHead && !res.headersSent) {
      res.writeHead(500);
    }
    res.end(
      'Proxy error: Could not proxy request ' +
        req.url +
        ' from ' +
        host +
        ' to ' +
        proxy +
        ' (' +
        err.code +
        ').'
    );
  };
}
function onProxyError ( proxy ) { return ( err , req , res ) => { const host = req . headers && req . headers . host ; console . log ( chalk . red ( 'Proxy error:' ) + ' Could not proxy request ' + chalk . cyan ( req . url ) + ' from ' + chalk . cyan ( host ) + ' to ' + chalk . cyan ( proxy ) + '.' ) ; console . log ( 'See https://nodejs.org/api/errors.html#errors_common_system_errors for more information (' + chalk . cyan ( err . code ) + ').' ) ; console . log ( ) ; // And immediately send the proper error response to the client. // Otherwise, the request will eventually timeout with ERR_EMPTY_RESPONSE on the client side. if ( res . writeHead && ! res . headersSent ) { res . writeHead ( 500 ) ; } res . end ( 'Proxy error: Could not proxy request ' + req . url + ' from ' + host + ' to ' + proxy + ' (' + err . code + ').' ) ; } ; }
We need to provide a custom onError function for httpProxyMiddleware. It allows us to log custom error messages on the console.
We need to provide a custom onError function for httpProxyMiddleware . It allows us to log custom error messages on the console .