Provide a truly immutable version of ReadonlyArray

[danielnixon]

ReadonlyArray is mutable for the same reason we provide ImmutableSet and ImmutableMap - method syntax.

Here's an example - this compiles:

const foo: ReadonlyArray<string> = ["asdf"] as const;

// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
foo.every = () => false;
// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
foo.at = () => undefined;

Unfortunately, due to what I assume is a special case rule (because arrays get special treatment by the compiler), Readonly does not fix the problem for ReadonlyArray the way it does for ReadonlySet and ReadonlyMap and any other type.

This still compiles:

// eslint-disable-next-line functional/type-declaration-immutability
export type ImmutableArray<T> = Readonly<ReadonlyArray<T>>;

const foo: ImmutableArray<string> = ["asdf"] as const;

// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
foo.every = () => false;
// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
foo.at = () => undefined;

If we want an ImmutableArray (i.e. one that isn't flagged by the type-declaration-immutability eslint rule), we will have to copy all of the ReadonlyArray methods from lib.es5.d.ts (and elsewhere probably) and replace them with readonly properties by hand.

[danielnixon]

In the meantime, let's configure this override: https://github.com/eslint-functional/eslint-plugin-functional/blob/main/docs/rules/settings/immutability.md#example-of-configuring-immutability-overrides

[danielnixon]

If we did go down the path of creating a copy of of the ReadonlyArray type by hand, that would be the time to solve this too https://github.com/agiledigital/readonly-types/issues/7 .

[danielnixon]

Other builtins/primitives are similarly "ignored" by Readonly. For example, these all compile:

type Foo = Readonly<number>;

const a: Foo = 1;

// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
a.toString = () => "";

type Foo = Readonly<bigint>;

const a: Foo = BigInt(1);

// eslint-disable-next-line functional/no-expression-statements, functional/immutable-data, functional/functional-parameters
a.toString = () => "";

[danielnixon]

Oh, the answer was right in front of my eyes, courtesy of is-immutable-type.

type ImmutableShallow<T extends {}> = {
  readonly [P in keyof T & {}]: T[P];
};