title: reduce is not a function at Object.sum
body: <p>I have this example of code</p>
  
  <pre><code>let numbers = [1, 2, 3]
  let summableNum = Object.assign({}, numbers, {
    sum: function() {
      return this.reduce(function(a, b) {
        return a + b
      })
    }
  })
  </code></pre>
  
  <p>the result is:</p>
  
  <pre><code>{
    "0": 1,
    "1": 2,
    "2": 3,
    sum: f
  }
  
  summableNum[0]  // 1
  </code></pre>
  
  <p>when i execute summableNum.sum(), i have this error</p>
  
  <blockquote>
    <p>Uncaught TypeError: this.reduce is not a function
        at Object.sum (:4:17)
        at :11:17</p>
  </blockquote>
  
  <p>expected result:</p>
  
  <pre><code>summableNum.sum() // 6
  </code></pre>
  
  <p>can you help me</p>