title: Adding a new JSON key from an existing JSON value
body: <p>I have the following JSON;</p>
  <pre><code>[
    {
      &quot;id&quot;: 1,
      &quot;start&quot;: &quot;2022-06-20&quot;,
      &quot;result&quot;: 24
    },
    {
      &quot;id&quot;: 2,
      &quot;start&quot;: &quot;2022-06-21&quot;,
      &quot;result&quot;: 56
    },
    {
      &quot;id&quot;: 3,
      &quot;start&quot;: &quot;2022-06-21&quot;,
      &quot;result&quot;: 78
    }
  ]
  </code></pre>
  <p>I'm wanting to add 2 new values to each array above using JQ, <code>dimension</code> and <code>date</code>, but <code>date</code> needs to be a copy of the existing key value <code>start</code>. The expected output is as below;</p>
  <pre><code>[
    {
      &quot;id&quot;: 1,
      &quot;start&quot;: &quot;2022-06-20&quot;,
      &quot;result&quot;: 24,
      &quot;date&quot;: &quot;2022-06-20&quot;,
      &quot;dimension&quot;: &quot;new&quot;
    },
    {
      &quot;id&quot;: 2,
      &quot;start&quot;: &quot;2022-06-21&quot;,
      &quot;result&quot;: 56,
      &quot;date&quot;: &quot;2022-06-21&quot;,
      &quot;dimension&quot;: &quot;new&quot;
    },
    {
      &quot;id&quot;: 3,
      &quot;start&quot;: &quot;2022-06-21&quot;,
      &quot;result&quot;: 78,
      &quot;date&quot;: &quot;2022-06-21&quot;,
      &quot;dimension&quot;: &quot;new&quot;
    }
  ]
  </code></pre>
  <p>The jq I have at present can add the new key <code>dimension</code>, but I can't figure out how to copy <code>start -&gt; date</code></p>
  <pre><code>jq '.[] += {&quot;dimension&quot;: &quot;new&quot;}' input.json
  </code></pre>
  <p>Thanks for any help</p>