Javascript - Do something when an element in two arrays are the same? -

i found solution returned array of elements without duplicates:

array1 = array1.filter(function(val) {   return array2.indexof(val) == -1; }); 

however, want modify code little bit. instead of being returned array without duplicates, want when there duplicate. problem is, i'm not sure how code works. thing i'm not sure how val gets set, or is.

for (var = 0; < json.length; i++) {      var item = json[i];     // if json.indexof(val?), } 

read docs array filter method then. val parameter of callback passed single array items, i.e. json[i] or item in case:

for (var = 0; < json.length; i++) {      var item = json[i];     if (json.indexof(item) >= 0) {         //     } }