arr1=[1,8,1,3,2]

 arr2=[3,8,1]

function subsetFind() {
    var arr1 = [1,8,1,3,2]
    var arr2 = [3,8,1]
    var arr3 = [];
    var arr1length = arr1.length;
    var arra2length = arr2.length;

   for(var i = 0; i < arr1length; ++i){
        for(var j=0;j<arra2length;j++) {
            if(arr1[i] != arr2[j]) {
                break;
            } else {
                arr3.push(arr1[i]);
                break;
            }
        }
    }

    alert(arr3);
}

function subsetFind() {
    var arr1 = [1,8,1,3,2]
    var arr2 = [3,8,1]
    var arr3 = [];
    var arr1length = arr1.length;
    var arra2length = arr2.length;
    var used_i = 0;
    for(var i = 0; i < arr1length; ++i){
        if(used_i != 0 && used_i < i-1){
            break;
        }
        for(var j=0;j<arra2length;j++) {
            if((arr1[i] == arr2[j] && arr1[i-1] == arr2[j-1]) || (arr1[i] == arr2[j] && arr1[i+1] == arr2[j+1])) {
               arr3.push(arr1[i]);
               used_i = i;
            }
        }
    }

    alert(arr3);
}

8,1

function subset () {
        var arr1 = [1,9,3,5,4,8,2,6,3,4]
        var arr2 = [5,2,4,8,2,6,4]
        var arr3 = [];
        var arr1length = arr1.length;
        var arra2length = arr2.length;
        var finalResult;
       for(var i = 0; i < arr1length; ++i){
            for(var j=0;j<arra2length;j++) {
                if((arr1[i] == arr2[j] && arr1[i-1] == arr2[j-1]) || (arr1[i] == arr2[j] && arr1[i+1] == arr2[j+1])) {
                    arr3.push(arr1[i]);

                }
                else
                {
                    finalResult = arr3.toString();
                }
            }
        }

        alert(finalResult);
}

[...new Set(arr1)].filter(v => arr2.includes(v))

new Set(arr1)                   // convert arr1 to Set to remove duplicates

[...new Set(arr1)]              // convert back to array to `filter`

arr2.includes(v)                // test if arr2 includes `v`

[...new Set(arr1)].filter(v => arr2.includes(v))  // choose unique elements in both arrays