SystemJS making ~200 requests for non-existing files

Question

I'm really lost about this. When I load my Angular2 app, I have about 200 requests to my server for non-existing files under node_modules/systemjs. Here's a sample of of those requests:

127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/Subject.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/Observable.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/root.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/toSubscriber.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/Subscriber.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/isFunction.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/Subscription.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/isArray.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/isObject.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/tryCatch.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/errorObject.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/UnsubscriptionError.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/Observer.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/rxSubscriber.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/observable.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/ObjectUnsubscribedError.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/SubjectSubscription.js.map HTTP/1.1" 404 -
127.0.0.1 - - [13/Sep/2016 13:00:06] "GET /node_modules/systemjs/dist/from.js.map HTTP/1.1" 404 -

My Angular2 application works fine, I don't get a single error, everything functions as expected. But those ~200 404 requests are really slowing down the page load. For a reason I really can't understand, those 404 requests don't even show up in my browser's network tab, but Wireshark confirms they are coming from there.

Here is my systemjs.config.js:

/**
 * System configuration for Angular 2 samples
 * Adjust as necessary for your application needs.
 */
(function (global) {
  System.config({
    defaultJSExtensions: true,
    paths: {
      // paths serve as alias
      'npm:': 'node_modules/'
    },
    // map tells the System loader where to look for things
    map: {
      // our app is within the app folder
      app: 'app',
      // angular bundles
      '@angular/core': 'npm:@angular/core/bundles/core.umd.js',
      '@angular/common': 'npm:@angular/common/bundles/common.umd.js',
      '@angular/compiler': 'npm:@angular/compiler/bundles/compiler.umd.js',
      '@angular/platform-browser': 'npm:@angular/platform-browser/bundles/platform-browser.umd.js',
      '@angular/platform-browser-dynamic': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic.umd.js',
      '@angular/http': 'npm:@angular/http/bundles/http.umd.js',
      '@angular/router': 'npm:@angular/router/bundles/router.umd.js',
      '@angular/forms': 'npm:@angular/forms/bundles/forms.umd.js',
      // angular testing umd bundles
      '@angular/core/testing': 'npm:@angular/core/bundles/core-testing.umd.js',
      '@angular/common/testing': 'npm:@angular/common/bundles/common-testing.umd.js',
      '@angular/compiler/testing': 'npm:@angular/compiler/bundles/compiler-testing.umd.js',
      '@angular/platform-browser/testing': 'npm:@angular/platform-browser/bundles/platform-browser-testing.umd.js',
      '@angular/platform-browser-dynamic/testing': 'npm:@angular/platform-browser-dynamic/bundles/platform-browser-dynamic-testing.umd.js',
      '@angular/http/testing': 'npm:@angular/http/bundles/http-testing.umd.js',
      '@angular/router/testing': 'npm:@angular/router/bundles/router-testing.umd.js',
      '@angular/forms/testing': 'npm:@angular/forms/bundles/forms-testing.umd.js',
      // other libraries
      'rxjs':                       'npm:rxjs',
      'angular2-in-memory-web-api': 'npm:angular2-in-memory-web-api',
      'moment':                     'npm:moment/moment',
      'ng2-bs3-modal':              'npm:ng2-bs3-modal',
    },
    // packages tells the System loader how to load when no filename and/or no extension
    packages: {
      app: {
        main: './main.js',
        defaultExtension: 'js'
      },
      rxjs: {
        defaultExtension: 'js'
      },
      'angular2-in-memory-web-api': {
        main: './index.js',
        defaultExtension: 'js'
      },
    }
  });
})(this);

Answer 1

What you are seeing is your browser requesting source maps, files that act as "dictionaries" that translate transformed code (like transpilation or minification) into its form prior to transformation. This usually kicks in when you have the debugger open and with source map support enabled. This is normal behavior. If you close your debugger and reload your app, source map requests will not happen.

However, it looks like your app is requesting for source maps in the wrong place. Those look like third-party libraries looking for their source map in the systemjs module directory. Refer to the systemjs documentation on how paths are handled.