client.js

const client = sanityClient({
  projectId: 'p354jyup',
  dataset: 'main',
  useCdn: true,
});

// needs polyfill
const origin = window.location.origin

client.clientConfig.cdnUrl = `${origin}/sanity`;
client.clientConfig.url = `${origin}/sanity`;

server.js

app.use(
  '/sanity',
  proxy('https://p354jyup.apicdn.sanity.io/', {
    https: true,
    proxyReqPathResolver: function(req: express.Request) {
      const parts = parseUrl(req.url);
      return `/v1${parts.pathname}?${parts.search || ''}`;
    },
  })
);