[danik.js]

И как тогда?

var a1=[1,2,3,4,5];
var a2=[2,4,4,2];

alert(isSubset(a1, a2));

function isSubset(a1, a2) {
    for (var i = 0; i < a2.length; i++) {
        var element = a2[i];
        var found = false;
        for (var j = 0; j < a1.length; j++) {
            if (a1[j] === element) {
                found = true;
                break;
            }
        }
        if (!found) {
            return false;
        }
    }
    return true;
}