title: function with .filter() returning undefined instead of filtered array
body: <p>I'm trying to understand javascript's <code>Array.filter</code> method. </p>
  
  <p>Why is the following code returning <code>undefined</code>? What am I missing?</p>
  
  <pre><code>function driversWithRevenueOver(driver, revenue) {
    driver.filter(function(person) {
      if (person.revenue &gt;= revenue) {
        return person;
      }
    });
  }
  
    driversWithRevenueOver(
      [
        { name: "Sally", revenue: 400 },
        { name: "Annette", revenue: 200 },
        { name: "Jim", revenue: 150 },
        { name: "Sally", revenue: 200 }
      ],
      250
    );
  </code></pre>
  
  <p>It should return:</p>
  
  <pre><code>[{ name: 'Sally', revenue: 400 }]
  </code></pre>