title: Get the sum of 3rd property if first two properties matches
body: <p>I have three properties in an array. I want the the sum of the 3rd property when the first 2 properties have the same value. My goal is to have an array in which the first two properties are unique.</p>
  
  <p>For the following example</p>
  
  <pre><code>var arrayPeople = [
      {
          name: 'joe',
          category: 'leave',
          count: 2,
      },
      {
          name: 'bob',
          category: 'travel',
          count: 1,
      },
      {
          name: 'joe',
          category: 'leave',
          count: 1,
      },
      {
          name: 'bob',
          category: 'travel',
          count: 3,
      };   
  ];
  </code></pre>
  
  <p>I would expect </p>
  
  <pre><code>arrayPeople = [
      {
          name: 'joe',
          category: 'leave',
          count: 3,
      },
      {
          name: 'bob',
          category: 'travel',
          count: 4,
      };
  ];
  </code></pre>
  
  <p>There is only one occurrence of <code>name: 'joe', category: 'leave'</code> and its <code>count: 3</code> is the sum of the two counts in the first array.</p>