Array.prototype.unique = function() {
    var a = this.concat();
    for(var i=0; i<a.length; ++i) {
        for(var j=i+1; j<a.length; ++j) {
            if(a[i] === a[j])
                a.splice(j, 1);
        }
    }
    return a;
};

var A = [1,2,3,4];
var B = [2,3,4,5];
var C = A.concat(B).unique();