title: what difference for ....loop syntax in javascript?
body: <pre><code> num = {num1 : 10 , num2 : 17, num3 : 22};
  </code></pre>
  
  <p>Case 1</p>
  
  <pre><code>for (let key in num){
                        alert(key);
  }
  </code></pre>
  
  <p>Case 2</p>
  
  <pre><code>for (key in num){
                        alert(key);
  }
  </code></pre>
  
  <blockquote>
    <p>key and let key </p>
  </blockquote>
  
  <p>what is diference and how we use it</p>