คำสั่ง alert ไม่ทำงานเมื่อข้าม index array เราจะใช้ if check ไงดีครับ

arr[1] = new Array("page1_start","page1_stop");
arr[3] = new Array("page3_start","page3_stop");
arr[4] = new Array("page4_start","page4_stop");
arr[6] = new Array("page6_start","page6_stop");



						 for(i=1; i<arr.length; i++)
						 {
						
							 if(arr[i]==arr[2]||arr[i]==arr[5] )  continue;
						 alert("start is: "+arr[i][0]);
					     alert("stop is: "+arr[i][1]);
						
						}

var arr = new Array();
arr[1] = new Array("page1_start P","page1_stop");
arr[3] = new Array("page3_start","page3_stop");

			for(i=1; i<arr.length; i++){

                                        if (typeof arr[i] != 'undefined'){
                     			alert("start is: "+arr[i][0]);
			                alert("stop is: "+arr[i][1]);
                                        }

		          }