title: TS2339: Property 'includes' does not exist on type 'string'
body: <p>I have seen this error mentioned in regards to string arrays but not actual strings. I have a TypeScript file with the line</p>
  
  <pre><code>if (!bus.lineInfo.PublishedLineName.includes(input)) {
  </code></pre>
  
  <p>This gives me an error of</p>
  
  <pre><code>TS2339: Property 'includes' does not exist on type 'string'.
  </code></pre>
  
  <p><code>bus</code> is a variable that implements the <code>bus</code> interface:</p>
  
  <pre><code>interface bus {
      "lineInfo": {
          "PublishedLineName": string,
          "DestinationName": string, // The headsign of the bus
          "Color": string,
          "TextColor": boolean | string // false if this is "FFFFFF", otherwise it's the color
      },
      "warnings": boolean | busWarnings
      "marker"?: google.maps.Marker,
      "result"?: JQuery // The search result that appears in the sidebar
  }
  </code></pre>
  
  <p><code>lineInfo.PublishedLineName</code> is declared as a <code>string</code>, and <code>String.prototype.includes()</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/includes" rel="noreferrer">is a function according to MDN</a>, so why does the TypeScript compiler complain about the missing property/method?</p>