title: Array.push() only to specific index of an object?
body: <p>I'm setting up an object with year objects, month arrays, and days that are arrays of post objects. When I try to push data to <code>postData[year][month][date]</code>, it pushes to all the date arrays in <code>month</code>. </p>
  
  <pre class="lang-js prettyprint-override"><code>let postData = {}
  
  const addPosts = (year, month, newPosts) =&gt; {
    postData[year] = postData[year] || {}
    postData[year][month] = postData[year][month] || Array(30).fill([])
    postData[year][month][0].push('post')
  
    return postData
  }
  </code></pre>
  
  <p>Outputs:</p>
  
  <pre><code>{
    "2020": {
      "4": [
        [
          "post"
        ],
        [
          "post"
        ],
        [
          "post"
        ],
        ...
      ]
    }
  }
  </code></pre>
  
  <p>Is there something about .push() I'm not understanding or is there a limit to the number of indexes you can .push() into?</p>