I would like to enforce function arguments to be read-only (immutable), to prevent me from accidentally mutate user input. I've encountered many modules that accidentally modify a user-supplied options-object.

I would also like to enforce function return values to not be read-only, as that's annoying for users without much benefit, since it's not actually enforced at runtime. This one might be slightly controversial, so could be an option.

Fail

function foo(input: number[]): void {}

function foo(input: string): Readonly<Point> {}

Pass

function foo(input: readonly number[]): void {}

function foo(input: string): Point {}

The rule should support enforcing read-only for:

• readonly (array and tuple)
• readonly (object property)
• Readonly (object properties)
• ReadonlyArray
• readonly for object keys: {readonly [user: string]: boolean}