When a request comes into ASP.NET Core, it goes through what is called the request/response pipeline, where some middleware code is executed. This pipeline is configured using a method called Configure. It is this method we will use to define exactly which middleware is executed and in what order. Middleware code is invoked by methods that generally start with Use in the app parameter. So, we would typically specify middleware such as authentication early in the Configure method, and MVC middleware toward the end. The pipeline that the template created is as follows:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    ...
    app.UseStaticFiles();
    app.UseSpaStaticFiles();
    app.UseRouting();
    app.UseEndpoints( ... );

    app.UseSpa(spa =>
    {
        spa.Options.SourcePath = "ClientApp";

        if (env.IsDevelopment())
        {
            spa.UseReactDevelopmentServer(npmScript: "start");
        }
    });
}

Notice that a method called UseSpaStaticFiles is called in the pipeline, just before the routing and endpoints are set up. This allows the host to serve the React app, as well as the web API.

Also, notice that a UseSpa method is called after the endpoint middleware. This is the middleware that will handle requests to the React app, which will simply serve the single page in the React app. It is placed after UseEndpoints so that requests to the web API take precedence over requests to the React app.

The UseSpa method has a parameter that is actually a function that executes when the app is first run. This function contains a branch of logic that calls spa.UseReactDevelopmentServer(npmScript: "start") if you're in development mode. This tells ASP.NET Core to use a development server by running npm start. We'll delve into the npm start command later in this chapter. So, in development mode, the React app will be run on a development server rather than ASP.NET Core serving the files from ClientApp/Build. We'll learn more about this development server later in this chapter.