[Preview 4] Blazor WASM HostEnvironment (#17856)

Author: guardrex

aspnetcore/blazor/hosting-model-configuration.md

@@ -60,6 +60,36 @@ Obtain the app's environment in a component by injecting `IWebAssemblyHostEnviro
 <p>Environment: @HostEnvironment.Environment</p>
 ```
 
+During startup, the `WebAssemblyHostBuilder` exposes the `IWebAssemblyHostEnvironment` through the `HostEnvironment` property, which enables developers to have environment-specific logic in their code:
+
+```csharp
+if (builder.HostEnvironment.Environment == "Custom")
+{
+    ...
+};
+```
+
+The following convenience extension methods permit checking the current environment for Development, Production, Staging, and custom environment names:
+
+* `IsDevelopment()`
+* `IsProduction()`
+* `IsStaging()`
+* `IsEnvironment("{ENVIRONMENT NAME}")
+
+```csharp
+if (builder.HostEnvironment.IsStaging())
+{
+    ...
+};
+
+if (builder.HostEnvironment.IsEnvironment("Custom"))
+{
+    ...
+};
+```
+
+The `IWebAssemblyHostEnvironment.BaseAddress` property can be used during startup when the `NavigationManager` service isn't available.
+
 ### Configuration
 
 As of the ASP.NET Core 3.2 Preview 3 release ([current release is 3.2 Preview 4](xref:blazor/get-started)), Blazor WebAssembly supports configuration from: