title: Object.keys using numbers in typescript
body: <pre><code>type Foo = { [key: number]: string }
  
  const foo: Foo = { 100: 'foo', 200: 'bar' }
  const sizes: number[] = Object.keys(foo)
  </code></pre>
  
  <p>Gives me:</p>
  
  <blockquote>
    <p><code>Type 'string[]' is not assignable to type 'number[]</code></p>
  </blockquote>
  
  <p>Why does Typescript assume <code>string[]</code>? And what is the correct way of creating an array of numbers here?</p>