 <p>I think that they mean <code>string</code>, <code>number</code>, <code>boolean</code> and <code>undefined</code> because when you assign them to another variable, you copy the value, not a pointer.</p> <pre><code>var a = 'abc'; var b = a; a = 'def'; // b === 'abc' var a = { b: 'abc' }; var b = a; a.b = 'def'; // b.b === 'def' </code></pre>