docs: Explain getHubFromCarrier in express

src/collections/_documentation/platforms/javascript/express.md

@@ -4,31 +4,32 @@ sidebar_order: 1010
 ---
 
 <!-- WIZARD -->
+
 Our Express integration only requires _@sentry/node_ to be installed then you can use it like this:
 
 ```javascript
 const express = require('express');
 const app = express();
-const Sentry = require('@sentry/node') ;
+const Sentry = require('@sentry/node');
 
-Sentry.init({ dsn:'___PUBLIC_DSN___' });
+Sentry.init({ dsn: '___PUBLIC_DSN___' });
 
 // The request handler must be the first middleware on the app
 app.use(Sentry.Handlers.requestHandler());
 
 app.get('/', function mainHandler(req, res) {
-    throw new Error('Broke!');
+  throw new Error('Broke!');
 });
 
 // The error handler must be before any other error middleware
 app.use(Sentry.Handlers.errorHandler());
 
 // Optional fallthrough error handler
 app.use(function onError(err, req, res, next) {
-    // The error id is attached to `res.sentry` to be returned
-    // and optionally displayed to the user for support.
-    res.statusCode = 500;
-    res.end(res.sentry + '\n');
+  // The error id is attached to `res.sentry` to be returned
+  // and optionally displayed to the user for support.
+  res.statusCode = 500;
+  res.end(res.sentry + '\n');
 });
 
 app.listen(3000);
@@ -55,4 +56,37 @@ app.use(Sentry.Handlers.errorHandler());
 // to
 app.use(Sentry.Handlers.errorHandler() as express.ErrorRequestHandler);
 ```
+
 <!-- ENDWIZARD -->
+
+## Working with Scopes and Hubs
+
+Because of pipeline-like nature of Express.js and it's middlewares, the user has to pass the scope through all the pipes and carry it over with the request. It's required in order to isolate requests from each other.
+
+If you want to set per-requests user data and send it over to Sentry, you need to use `getHubFromCarrier` on the `request` object, instead of directly calling methods like `configureScope` or `captureException`. Here are some examples:
+
+```javascript
+app.get('/foo', (req, res, next) => {
+  Sentry.getHubFromCarrier(req).configureScope(scope => {
+    scope.setTag('foo', 'my-value');
+  });
+
+  throw new Error('foo');
+});
+
+app.get('/foo', (req, res, next) => {
+  Sentry.getHubFromCarrier(req).configureScope(scope => {
+    scope.setTag('foo', 'my-value');
+  });
+
+  if (someCheck()) {
+    Sentry.getHubFromCarrier(req).captureMessage('Invalid eventId');
+    next();
+  } else {
+    res.end();
+  }
+});
+```
+
+All `configureScope` calls that were done directly on initialized `Sentry` object, will be carried to each request, so it's a good way to set some data globally.
+