title: setting a type only for the value of an object
body: <p>I have this object:</p>
  <pre><code>const theme = {}
  </code></pre>
  <p>This object is being filled by a function, this function adds a property to this object whenever it's called, the key it sets is a random number as a string, so &quot;1&quot;, &quot;31&quot; and so on, and the <strong>value</strong> it sets is a string.</p>
  <p>I was looking for something like this in TypeScript:</p>
  <pre><code>interface Theme {
      string: string
  }
      
  const theme: Theme = {}
  
  const theFunction(){
     const key = `${Math.floor(Math.random() * 1000)}`;
     const value = &quot;value&quot;;
     theme[key] = value;
  }
  </code></pre>
  <p>The <strong>theFunction</strong> might get called at any unknown time,</p>