Implement and document asp-prerender-data

src/Microsoft.AspNetCore.SpaServices/README.md

@@ -81,6 +81,34 @@ If you try running your app now, you should see the HTML snippet generated by yo
 
 As you can see, your JavaScript code receives context information (such as the URL being requested), and returns a `Promise` so that it can asynchronously supply the markup to be injected into the page. You can put whatever logic you like here, but typically you'll want to execute a component from your Angular 2 / React / etc. application.
 
+**Passing data from .NET code into JavaScript code**
+
+If you want to supply additional data to the JavaScript function that performs your prerendering, you can use the `asp-prerender-data` attribute. You can give any value as long as it's JSON-serializable. Bear in mind that it will be serialized and sent as part of the remote procedure call (RPC) to Node.js, so avoid trying to pass massive amounts of data.
+
+For example, in your `cshtml`,
+
+    <div id="my-spa" asp-prerender-module="ClientApp/boot-server"
+                     asp-prerender-data="new {
+                        IsGoldUser = true,
+                        Cookies = ViewContext.HttpContext.Request.Cookies
+                    }""></div>
+
+Now in your JavaScript prerendering function, you can access this data by reading `params.data`, e.g.:
+
+```javascript
+module.exports = function(params) {
+    return new Promise(function (resolve, reject) {
+        var result = '<h1>Hello world!</h1>'
+            + '<p>Is gold user: ' + params.data.isGoldUser + '</p>'
+            + '<p>Number of cookies: ' + params.data.cookies.length + '</p>';
+
+        resolve({ html: result });
+    });
+};
+```
+
+Notice that the property names are received in JavaScript-style casing (e.g., `isGoldUser`) even though they were sent in C#-style casing (e.g., `IsGoldUser`). This is because of how the JSON serialization is configured by default.
+
 **Passing data from server-side to client-side code**
 
 If, as well as returning HTML, you also want to pass some contextual data from your server-side code to your client-side code, you can supply a `globals` object alongside the initial `html`, e.g.:
@@ -245,7 +273,7 @@ At this stage, run `webpack` on the command line to build `wwwroot/dist/main.js`
 
 You can now run your React code on the client by adding the following to one of your MVC views:
 
-    <div id="my-spa"></div>
+    <div id="my-spa"></div>
     <script src="/dist/main.js"></script>
 
 #### Running React code on the server