title: Array.prototype.reduce() Error After Including Initial Value
body: <p>I am using <code>reduce()</code> in one of my functions to get a single value from some documents. Generally, this works as expected. However, once in a while I get an error "reduce is not a function". I know that, according to the docs, "Calling reduce() on an empty array without an initial value is an error" -- so I am passing "0" to handle that. But I still sometimes - though rarely - get the error. Just trying to understand what's going on.</p>
  
  <p>My code looks like this:</p>
  
  <pre><code> let totalOpenBalance = docs.reduce((a, curr) =&gt; a + curr.openBalance, 0);
  </code></pre>
  
  <p>The data I'm calling this on looks like this:</p>
  
  <pre><code>[
      {
          "_id": "3s14020471287f4a9bbe9878",
          "openBalance": 120,
          "field3": 25
      },
      {
          "_id": "2d75020471986f4a9bbe7231",
          "openBalance": 15,
          "field3": 22
      }
  ]
  </code></pre>