title: Typescript, NodeJS, ES6 - findIndex on array
body: <p>So i have an issue with using findIndex ES6 function on an array in typescript and NodeJS.</p>
  
  <p>The error message i'm getting is:</p>
  
  <blockquote>
    <p>requestType.findIndex is not a function</p>
  </blockquote>
  
  <p>this is my code:</p>
  
  <pre><code>let weatherStates:Array&lt;string&gt; = ['current', 'forecast'];
  let requestType:string = 'current';
  requestType.findIndex(weatherStates);
  </code></pre>
  
  <p>and then the error gets thrown, i'm able to use all sorts of other ES6 functionalities though, like the arrow functions etc.</p>
  
  <p>it looks like that the polyfill is available in the lib.core.d.ts typing though.</p>
  
  <p>Here is mytsconfig.json setup:</p>
  
  <pre><code>{
  "compilerOptions": {
      "module": "commonjs",
      "target": "es6",
      "sourceMap": true
  },
  "exclude": [
      "node_modules",
      "typings"
  ]
  }
  </code></pre>
  
  <p>i'm fairly new with typescript so any help would be greatly appreciated!</p>