Add a rule that requires to use $ReadOnlyArray instead of Array

[gajus]

Rule must warn when Array is used instead of $ReadOnlyArray.

[pnevyk]

@gajus I am working on this. And I have basically three questions:

• Should this rule apply also on string[] notation? I would guess so. It should be probably configurable.
• Does this rule need any specific configuration (apart from one mentioned above)? For example to specify exceptions for this rule somehow, or any other.
• Do you have a nice text about reason for this use? I think it should be noted in the docs. I found this general article about immutability and some Flow issues where $ReadOnlyArray solves problems with Array (1, 2).

[gajus]

Should this rule apply also on string[] notation?

Yes. Regardless of whether the array contains primitive data or not, it is mutable. There is no short-hand syntax for non-mutable array.

Does this rule need any specific configuration (apart from one mentioned above)? For example to specify exceptions for this rule somehow, or any other.

I cannot think of any exceptions.

It should always require to use $ReadOnlyArray. When user intends to use Array, he should use // eslint-disable-next-line require-read-only-array.

Do you have a nice text about reason for this use? I think it should be noted in the docs. I found this general article about immutability and some Flow issues where $ReadOnlyArray solves problems with Array (1, 2).

All generic pro-immutable data arguments apply, e.g.

• immutable objects are simpler to construct, test, and use
• they help to avoid temporal coupling
• their usage is side-effect free (no defensive copies)
• identity mutability problem is avoided
• they always have failure atomicity
• they are much easier to cache

– https://softwareengineering.stackexchange.com/questions/151733/if-immutable-objects-are-good-why-do-people-keep-creating-mutable-objects

[pnevyk]

@gajus This is already merged. You can close this issue.