title: Js object key: plus value syntax
body: <p>I found this weird syntax in <a href="http://bl.ocks.org/tjdecke/5558084" rel="nofollow">d3 example</a></p>
  
  <pre><code>var heatmapChart = function(tsvFile) {
      d3.tsv(tsvFile,
      function(d) {
        return {
          day: +d.day,
          hour: +d.hour,
          value: +d.value
        };
      },
  </code></pre>
  
  <p>What plus in </p>
  
  <pre><code>      day: +d.day,
  </code></pre>
  
  <p>mean?</p>