今天刚看到js归并排序排序有一段知道作用和Math.floor()一样，~是取反的意思，两次取反还是什么的，之前在jQuery源码里面还见到类似 + function(){}、?function(){}、!function(){}.这样的目的是什么，让源码看起来更加简单，效率更高还是怎么？求教！！！

function merge(left, right) {
  var tmp = [];

  while (left.length && right.length) {
    if (left[0] < right[0])
      tmp.push(left.shift());
    else
      tmp.push(right.shift());
  }

  return tmp.concat(left, right);
}

function mergeSort(a) {
  if (a.length === 1) 
    return a;

  var mid = ~~(a.length / 2) //------------------->"~~"
    , left = a.slice(0, mid)
    , right = a.slice(mid);

  return merge(mergeSort(left), mergeSort(right));
}