var getRandomItem = function(arr) {

  return arr[Math.floor(Math.random() * arr.length)];

}

​

// original array

var arr = [4, 3, 1, 6, 9, 8, 5];

​

// number of random elements to get from arr

var n = 4;

​

if (n > arr.length) {

  alert('n must be less than or equal arr length');

}

​

var count = 0;

// new array to push random item in

var randomItems = []

do {

  var item = getRandomItem(arr);

  randomItems.push(item);

  // update the original array and remove the recently pushed item

  arr.splice(arr.indexOf(item), 1);

  count++;

} while(count < n);

​

console.log(randomItems);

console.log(arr);

​