Javascript array concatenation

gistfile1.js

// Following concat function concatenate javascript arrays
// It consider three more factor then normal Array.prototype.concat:
// 1. eliminate common element
// 2. escape null and undefined
// 3. deal with element being an array
var arr1 = ['a'];
var arr2 = ['b', 'c'];
var arr3 = ['c', ['d'], 'e', undefined, null];

// return object classname
var classof = function(o){
  if (o === null) { return "null"; }
  if (o === undefined) { return "undefined"; }
  var className = Object.prototype.toString.call(o).slice(8,-1);
  return className;
};

var concat = (function(){
  // concat arr1 and arr2 without duplication.
  var concat_ = function(arr1, arr2) {
    for (var i=arr2.length-1;i>=0;i--) {
      // escape undefined and null element
      if (arr2[i] === undefined || arr2[i] === null) {
        continue;
      }
      // recursive deal with array element
      // can also handle multi-level array wrapper
      if (classof(arr2[i]) === 'Array') {
        for (var j=arr2[i].length-1;j>=0;j--) {
          concat_(arr1, arr2[i][j]);
        }
        continue;
      }
      arr1.indexOf(arr2[i]) === -1 ? arr1.push(arr2[i]) : 0;
    }
  };
  // concat arbitrary arrays.
  // Instead of alter supplied arrays, return a new one.
  return function(arr) {
    var result = arr.slice();
    for (var i=arguments.length-1;i>=1;i--) {
      concat_(result, arguments[i]);
    }
    return result;
  };
}());

console.log( arr1.concat(arr2, arr3) );
// result: [ 'a', 'b', 'c', 'c', [ 'd' ], 'e', undefined, null ]
console.log( concat(arr1, arr2, arr3) );
// result: [ 'a', 'e', 'd', 'c', 'b' ]
delete arr3[2];
console.log( concat(arr1, arr2, arr3) );
// result: [ 'a', 'd', 'c', 'b' ]