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JavaScript

//True array
['a','b','c'].concat(['4','5','6']); //["a", "b", "c", "4", "5", "6"]

//arguments is not a true array
function fakeArray(){
    return ['a','b','c'].concat(arguments);
}
fakeArray('4','5','6'); //["a", "b", "c", { 0:'4', 1:'5', 2:'6'}]