Array.prototype.difference

Array.prototype.difference = function(arr) {
    var flag = (arr.length > this.length);
    var tempArr1 = flag ? arr.slice(0) : this.slice(0);
    var tempArr2 = flag ? this.slice(0): arr.slice(0);
    var newArr = [];
    tempArr1.forEach(function(val, pos) {
        var index = tempArr2.indexOf(val);
        if(index > -1)
            tempArr2.splice(index, 1);
        else
            newArr.push(val);
    });
    return newArr.concat(tempArr2);
};