Calling reduce to sum array of objects returns NaN
<p>I have code similar to this:</p>
<p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
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<pre class="snippet-code-js lang-js prettyprint-override"><code>var temp = [ { "y": 32 }, { "y": 60 }, { "y": 60 } ];
var reduced = temp.reduce(function(a, b) {
  return a.y + b.y;
});

console.log(reduced); // Prints NaN</code></pre>
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</p>
<p>Why does it print <code>NaN</code> instead of <code>152</code>?</p>