title: Add key and incremental values to array of objects
body: <p>I have an array of objects. I want to add another property to each of the objects but I want the value to be incremental in nature. I want each to be 5 more than the other.</p>
  <p>My array of objects:</p>
  <pre><code>let arr_obj = [
     {
       name: 'Hermione',
       order: 'books',
     },
     {
       name: 'Harry',
       order: 'brooms',
     },
     {
       name: 'Ron',
       order: 'food',
     }
  ]
  </code></pre>
  <p>I want to add another property to each of the objects in the array and want the value to be 25 less the previous one.</p>
  <p>So what I want my object to look like (expected result):</p>
  <pre><code>arr_obj = [
     {
       name: 'Hermione',
       order: 'books',
       order_size: 100,
     },
     {
       name: 'Harry',
       order: 'brooms',
       order_size: 75, //100-25  
     },
     {
       name: 'Ron',
       order: 'food',
       order_size: 50, //75-25   
     }
  ]
  </code></pre>
  <p>I tried a forEach but that is not working. It does not subtract the 25.</p>
  <pre><code>let order_size = 100;
  arr_obj.forEach(d =&gt; {
     d['order_size'] = order_size-25;
  });
  </code></pre>
  <p>When I do this, I am getting:</p>
  <pre><code>arr_obj = [
     {
       name: 'Hermione',
       order: 'books',
       order_size: 100,
     },
     {
       name: 'Harry',
       order: 'brooms',
       order_size: 100,
     },
     {
       name: 'Ron',
       order: 'food',
       order_size: 100,
     }
  ]
  </code></pre>
  <p>How do I get the expected result?</p>