How does spa.UseAngularCliServer() middleware serve a webpage?

1. There is no need for a dist folder as the files are built and held in-memory by webpack dev server which is used internally by Angular CLI.

2. The requests for the SPA are reverse proxied to the Angular Dev Server by IIS. So IIS receives a request from the browser --> IIS sends a request to angular dev server --> dev server sends data back to IIS --> IIS sends data to browser.

Although that's done by proxying requests to the Angular Development Server , the proxying is not proxied by IIS , but by ASP.NET Core itself . IIS has no idea about the port listened by Angular Dev Server .

1. UseAngularCliServer("start") will firstly invoke the command

   npm start -- --port {dynamic_port}
   

The {dynamic_port} here is computed at RunTime .Since the npm command has been already configured in package.json , it will then start a dev server byng serve -- --port {dynamic_port} .

2. if the ng serve run flawlessly , it will create a plain HttpClient for proxy and use it to perform proxy (forward requests to Angular Dev Server with the plain HttpClient , and copy the response as it's own response) . The WebSocket proxy will take place in a similar way .

You can find the source code on GitHub . It's easy to read and understand .

Internally , the UseAngularCliServer(npmScript: "start") will invoke AngularCliMiddleware to do step 1 and step 2 . The step 1 is done by :

var angularCliServerInfoTask = StartAngularCliServerAsync(sourcePath, npmScriptName, logger);

The StartAngularCliServerAsync method will find an available TCP port , and run command npm start -- --port {port} to start ng serve :

private static async Task<AngularCliServerInfo> StartAngularCliServerAsync(
    string sourcePath, string npmScriptName, ILogger logger)
{
    var portNumber = TcpPortFinder.FindAvailablePort();
    logger.LogInformation($"Starting @angular/cli on port {portNumber}...");
    var npmScriptRunner = new NpmScriptRunner( sourcePath, npmScriptName, $"--port {portNumber}", null); 
    // ...
}

See the complete code here

And the Step 2 is done by :

public static void UseProxyToSpaDevelopmentServer(this ISpaBuilder spaBuilder, Func<Task<Uri>> baseUriTaskFactory)
{
    // ...
    // Proxy all requests to the SPA development server
    applicationBuilder.Use(async (context, next) =>
    {
        var didProxyRequest = await SpaProxy.PerformProxyRequest( context, neverTimeOutHttpClient, baseUriTaskFactory(), applicationStoppingToken, proxy404s: true);
    });
}

The implementation of PerfromProxyRequest() can be found here . For normal requests , It simply forward them using a plain HttpClient and copy the response as its own response .

Back to your question :

Where is the dist folder?

Since you have configured a spa.UseAngularCliServer("start"):

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

All those requests will be processed by local dev server launched by angular cli. As a result , there's no dist on disk at all . The same will take place if you run the command ng serve manually .

How does "ng serve" relate to IIS Express?

IIS has no idea about the port listened by Angular Dev Server. When there's an incoming message , IIS just sends it to your ASP.NET Core server . If the request cannot be processed by staticFiles , MVC , or any other middlewares that you've configured before UseSpa() , ASP.NET Core will forward it to the Dev Server .