JS Bin// source https://jsbin.com/loxeroh

threeSumClosest.js

/**
*/
"use strict";

var threeSumClosest = function threeSumClosest(num, target) {
  var result = num[0] + num[1] + num[num.length - 1];
  num.sort();
  for (var i = 0, il = num.length - 2; i < il; i++) {
    var start = i + 1;
    var end = num.length - 1;
    while (start < end) {
      var sum = num[i] + num[start] + num[end];
      if (sum > target) {
        end--;
      } else {
        start++;
      }
      if (Math.abs(sum - target) < Math.abs(result - target)) {
        result = sum;
      }
    }
  }
  return result;
};

console.log(threeSumClosest([-1, 2, 1, -4], 1));