Efficient pure Javascript array union

union.js

var array1 = [1,2,3,4,5];
var array2 = [1,2,3,4,5,6,7,8,9,10];
var union = [];
set = {};
for(var j = 0; j < array1.length; j++){
    set[array1[j]] = true;
}
for(var k = 0; k < array2.length; k++){
    set[array2[k]] = true;
}
for (var item in set){
    union.push(item);
}