[Hekumok]

с повторами

Array.prototype.random = function(n) {
 var l = this.length,
 newArr = [] ;
 while(n--)
  newArr.push(this[Math.floor(Math.random() * l)]) ;
 return newArr ;
} ;
var arr = [1, 2, 3, 4, 5] ;
alert(arr.random(3)) ;

Без повторов

Array.prototype.random = function(n) {
 var oldArr = this,
 newArr = [] ;
 while(n--) {
  newArr.push(oldArr.splice(Math.floor(Math.random() * oldArr.length), 1)[0]) ;
 } ;
 return newArr ;
} ;
var arr = [1, 2, 3, 4, 5] ;
alert(arr.random(3)) ;