title: terser Webpack plugin margle unwanted props
body: <p>I am using treser webpack plugin with :</p>
  <pre><code>      optimization: {
          minimize: true,
          minimizer: [new TerserPlugin({
            top_level: true,
            mangle: {
            //properties: true
             reserved:['contact'],
            keep_classnames: false,
           keep_fnames: false
            }
          })]
      }
  </code></pre>
  <p>My goal is to remove func names from the prototype with typescript code to one js bundle that works fine.
  However, I saw that when my code has variable:
  this.contact = {firstname:'user'}
  its compiled to contact.a and not contact.firstname.
  I dont want to margle this saved var.
  How can I do it and not destroy nested object..</p>