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re: JavaScript array methods: Mutator VS Non-mutator and the returning value VIEW POST


I think forEach is incorrectly categorized as a mutator function, since it does not modify the array itself, it only might modify the state of the objects inside the array. Actually, most of the non-mutator functions can also modify the state of the objects, for example map and filter.


Nice overview. But yes, I would not say .forEach() mutates too, since it is just kind of loop function - passed function may or may not mutate array on which it runs, it could be used for some side effects, it could be used to modify totally different array/object - it is just a loop over items in array and it does not imply any kind of function / computation in that loop.


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 =>x**x), since>x*x) won't change arr itself, it is new array, same as .filter.

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