title: ES6 copy properties to target only if they already exist
body: <p>Essentially what I want</p>
  
  <pre><code>let schema = {
         name: null,
         lastname: null
      }
  
  let values = {
         name: "John",
         unwanted: "haxor"
      }
  </code></pre>
  
  <p>to end up in: </p>
  
  <pre><code>console.log(sanitized); // {name: "John", lastname: null}
  </code></pre>
  
  <p>-- </p>
  
  <p>Using <code>Object.assign(schema, values)</code> ends up with the unwanted value. </p>
  
  <p>Is there a simple way?</p>
  
  <p>Edit: I should add, this is to avoid using a library like lodash or underscore, so if the solution is overly complex, they would be preferable solutions. </p>