title: custom typings files to angular 6
body: <p>I am trying to ad some custom typings definition file in angular  but facing some error.</p>
  
  <p>in app folder I have external folder in which I have external.d.ts file and below code</p>
  
  <pre><code>declare function format(input: string, ...args): string;
  </code></pre>
  
  <p>in tsconfig.json I tried adding 
  "files": ["/app/external/external.d.ts"]</p>
  
  <p>but its not working. </p>
  
  <p>can somebody give me idea about adding custom typings file to angular 6.</p>