title: Angular InjectionToken; what is 'app.config'?
body: <p>I am following the docs on how to provide, then inject a non-class instance; specifically I want to provide application wide constants.  <a href="https://angular.io/guide/dependency-injection#injectiontoken" rel="nofollow noreferrer">The docs</a> use the <code>InjectionToken</code> for this, but the explanations are very bare; consider the first part:</p>
  
  <p><strong>app.config.ts</strong></p>
  
  <pre><code>import { InjectionToken } from '@angular/core';
  
  export let APP_CONFIG = new InjectionToken&lt;AppConfig&gt;('app.config');
  </code></pre>
  
  <p>What's the point of the <em>'app.config'</em> string? Every example of InjectorToken that I see here uses that, but none discusses what it means or refers to.</p>