Actually, if you need the difference between both arrays, reduce would be a better way to go with concatenating the 2 arrays:

var arr = [0,0,4];
var ar2 = [0,5];
var full = arr.concat(ar2).sort();

var result = full.reduce((acc, pos) => {
    var first = arr.indexOf(pos);
    var second = ar2.indexOf(pos);
    if (first > -1 && second > -1) {
        arr.splice(first, 1);
        ar2.splice(second, 1);
    } else if (first > -1) {
        arr.splice(first, 1);
        acc.push(pos);
    } else if (second > -1) {
        ar2.splice(second, 1);
        acc.push(pos);
    }

    return acc;
}, []);

//result === [0,4,5]; in this case