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function addtwo(num) {
    return num + 2;
}

function timestwo(num) {
    return num * 2;
}

function arrplustwo(lst) {
    return lst.map(addtwo);
}

function mapper(lst, func) {
    for(var num in lst) lst[num] = func(lst[num]);
    return lst;
}

/*
   var t = [1, 2, 3, 4];
   mapper(t, timestwo)
=> [ 2, 4, 6, 8 ]*/