title: tslint: no-object-mutation while building Object with reduce
body: <p>I want to build an object using <code>reduce</code> in this way:</p>
  
  <pre><code>const result = [1, 2].reduce((partialResult, actualValue) =&gt; {
      // define someKey and someValue
      partialResult[someKey] = someValue;
      return partialResult
  }, {});
  </code></pre>
  
  <p>However, I'm getting the following tslint error:</p>
  
  <pre><code>Modifying properties of existing object not allowed. (no-object-mutation)
  </code></pre>
  
  <p>How can I solve this?</p>