Comment by mediumdeviation

Updating an array element

  arr[3] = 'New content';

Updating an array element immutably

  arr = arr.map((element, index) => {
    if (index === 3) return 'New content';
    return element;
  });

It's possible, sure, but it feels terrible. JavaScript objects want to be mutated - it's the most idiomatic way of interacting with them. Whether functional programming paradigm is good (I certainly feel it is) is beside the point - JS wasn't designed for it.

When I think of immutable primitives, I think of Python's tuples, which will raise a big fat error when you try to mutate them, or even better, Swift's structs, which gives you a new copy with every mutation.