ForEach itself does not mutate the array. But the function passed as parameter can mutate the array. And mostly forEach is used with a function parameter therefore it's more a mutator than a non-mutator. For the others, map and filter they both return a new array and reduce return a single value. But overall they does not mutate the original array.
You pass a function to map and filter as well, so they are also able to change the state of the object. Obviously, it's a bad practice to do so but still possible.
You have to reassign e.g. arr = arr.map(x=>x**x), since arr.map(x=>x*x) won't change arr itself, it is new array, same as .filter.
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