[prefer-readonly-parameter-types] Using prefer-readonly-parameter-types rule with lib.dom

[denis-afanasev]

I can't figure out how to use prefer-readonly-parameter-types rule on interop with DOM (lib.dom typings).

Previously i using @typescript-eslint/eslint-plugin: ^2.20.0 and have code like this:

class SomeClass {
...
    public someFunc(element: HTMLElement): void {
        ...
    }
}

After update to 2.23.0 i got a eslint error
Parameter should be a read only typeeslint(@typescript-eslint/prefer-readonly-parameter-types)
This rule require something like

class SomeClass {
...
    public someFunc(element: Readonly<HTMLElement>): void {
        ...
    }
}

After that i got next error on the same place Make all properties in T readonly.
It means that i should modify external library's code to fix that error, but it's impossible

I think that lib.dom (HTMLElement, etc) just 1 out of many errors from this rule on project-library interop layer.

This error caused by #1513
So I suggest to switch rule level from error to warn in packages/eslint-plugin/src/configs/all.json

[bradzacher]

See #1668
This is intentional - the rule enforces that objects are deeply readonly, not just surface readonly.

Ie HTMLElement et al are not deeply readonly, even with Readonly<>.
As this code is valid:

declare const x: Readonly<HTMLElement>;
x.classList = 1; // error assigning to readonly property
x.classList.foo = 1; // no readonly error, as `classList` is not readonly.

This is listed in the rule's docs:
https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/prefer-readonly-parameter-types.md#rule-details

A type is considered readonly if:
...

• it is an object type whose properties are all marked as readonly, and whose values are all considered readonly.

You will need to use a deep readonly type for these instances.