Макс

var a = [1, 2, 3, 4, 5],
    b = [1, 2, 3, 4, 5, 6];

function diff (a, b) {
    if (a.length > b.length) {
        return _.difference(a, b);
    } else {
        return _.difference(b, a);
    }
}

console.log(_.difference(b, a)); // [6]

function diff(a1, a2) {
    return a1.filter(i=>a2.indexOf(i)<0)
    .concat(a2.filter(i=>a1.indexOf(i)<0))
}
function compare(a1, a2) {
    return a1.length == a2.length && a1.every((v,i)=>v === a2[i])
}

const diff = function(a1, a2) {
    return a1.filter(i=>!a2.includes(i))
    .concat(a2.filter(i=>!a1.includes(i)))
}