title: concat 3 arrays into 1 array in javascript / underscore
body: <p>Is there a short way / best practice to concat 3 arrays into 1 array ?</p>
  
  <pre><code>var arr = [],
      arr1 = [ 1 , 2 , 3 ],
      arr2 = [ 4 , 5 , 6 ],
      arr3 = [ 7 , 8 , 9 ];
  arr = arr.concat(arr1);
  arr = arr.concat(arr2);
  arr = arr.concat(arr3);
  </code></pre>