Array intersect in javascript

intersect.js

Array.prototype.intersect =
  function() {
    if (!arguments.length)
      return [];
    var a1 = this;
    var a = null;
    var res = []
    var n = 0;
    var b = {};
    for(var i =0; i<a1.length;i++){
      b[a1[i]] = a1[i];
    }
    var n = 0;
    while(n < arguments.length) {
      a = arguments[n];
      for(i =0; i<a.length;i++){
          if (b[a[i]]){
            res.push(a[i]);
          }
      }
      n++;
    }
    
    return res;
  };