var threeSumClosest = function(nums, target) {
  var len = nums.length, sum=0;
  if(len===0) return 0;
  if(len<=3) {
    for(var i=0; i<len; i++){
        sum+=nums[i];
    }
    return sum;
  }
  nums.sort(function(a,b){
      return a-b;
  });

  console.log(nums);

  var closest = nums[0]+nums[1]+nums[2];
  for(var i=0; i<len; i++){
    for(var j=i+1, k=len-1; j<len-1, j<k; ){
      console.log(i + ', ' + j + ', ' + k + ': ' + nums[i]+ ', ' + nums[j] + ', ' + nums[k]);
      console.log('target:' + target + ', sum = ' + sum + ', difference: ' + (sum-target));
      var sum = nums[i] + nums[j] + nums[k];
      if(sum === target){
          return sum;
      } else if(sum<target){
        if((closest<sum && sum<target) ||
           Math.abs(target-sum)<Math.abs(target-closest)){
            closest = sum;
        }
        j++;
      } else if(sum>target){
        if(closest>sum && sum>target ||
           Math.abs(target-sum) < Math.abs(target-closest)) {
            closest = sum;
        }
        k--; //to reduce sum decrease tail
      }

    }
  }
  return closest;
};