title: Add missing properties to an object with a blank value if they don't exist
body: <p>I have an array of objects with different properties:</p>
  
  <pre><code>var arr = [];
  
  var obj_1 = {
      'key_a': 'value_a',
      'key_b': 'value_b'
  };
  
  arr.push(obj_1);
  
  var obj_2 = {
      'key_a': 'value_a',
      'key_b': 'value_b',
      'key_c': 'value_c'
  };
  
  arr.push(obj_2);
  </code></pre>
  
  <p>How do I automatically add the new key (key_c), or any other key, to every object in the array with a blank value if it doesn't already exist?</p>
  
  <p>So the array would look like this:</p>
  
  <pre><code>    [
          {
              'key_a': 'value_a'
              'key_b': 'value_b'
              'key_c': ''
          },
          {
              'key_a': 'value_a'
              'key_b': 'value_b'
              'key_c': 'value_c'
          }
      ]
  </code></pre>
  
  <p><strong>Please note:</strong> I'm building a web scraper and unknown keys/properties will be added to objects and pushed to the array in a foreach loop.</p>