for in

var fruits1 = new Array("apple" ,"banana", "orange");

for(var i=0;i <fruits1.length; i++){

}

이것과 

 

 

var fruits1 = new Array();

fruits[0]="apple";

fruits[1]="banana";

fruits[2]="orange";

 

for (var i in fruits1){

document.write(i)

}

이것은 같다