title: Vue.js bind to a class according to a condition
body: <p>With this div declaration:</p>
  <pre><code>&lt;div v-bind:class=&quot;[currentPage === 'help' ? highlight : '']&quot;&gt;
  </code></pre>
  <p>that I build accordingly to: <a href="https://v2.vuejs.org/v2/guide/class-and-style.html#Array-Syntax" rel="nofollow noreferrer">https://v2.vuejs.org/v2/guide/class-and-style.html#Array-Syntax</a></p>
  <p>But the binding isn't working (the class 'highlight' never applied regardless of the <code>currentPage</code> value).</p>
  <p>I have this model where there is the <code>currentPage</code> variable keeping track of the active page:</p>
  <pre><code>    var vueApp = new Vue({
          el: '#vueApp',
          data: {
              currentPage: 'help',
  </code></pre>
  <p>How can I activate the binding of a class on an element, based on the string value of a vue property?</p>