title: Typescript: How to make a type that is the keys of an interface but only the keys that are strings
body: <p>Suppose I have this interface</p>
  <pre class="lang-js prettyprint-override"><code>interface MyInterface {
     val1: string
     val2: string
     val3: number
     val4: string
     val5: Date
  }
  </code></pre>
  <p>I want to make a type that is the keys of the MyInterface but only the keys that are strings:</p>
  <pre class="lang-js prettyprint-override"><code>type myType = keyof MyInterface //where key === typeof 'string'
  // myType: 'val1' | 'val2' | 'val4'
  </code></pre>
  <p>How could I do something like this?</p>