Passing an immutable value Immutable<T> to a function that expects a mutable parameter T

[alexamy]

I frequently pass values of type Immutable<T> to functions that expect parameters of type T, and this usually works without any typing errors. However, yesterday I encountered a case where this didn't work (see code below).

As a workaround, I avoid the issue by passing problematic types together with Primitive | AtomicObject types to prevent the Immutable type from being applied to them in the Immutable type definition.

1. What is the intended type relationship between Immutable<T> and T?
2. Are they designed to be type-compatible, have I been observing some edge case or bug?
3. Are there any better solutions to this problem?

// Expecting to pass `Immutable<HTMLElement>` as well
const f = (element: HTMLElement) => {};

const el: Immutable<HTMLElement> = document.body;

// TypeScript error:
// The type 'readonly ImmutableObject<CSSStyleSheet>[]' is 'readonly' and cannot be assigned to the mutable type 'CSSStyleSheet[]'.
f(el);

[unadlib]

hi @alexamy,

Immutable is a strict immutable generic type that primarily uses TypeScript's readonly keyword to ensure properties are immutable, and it traverses all deep nested properties.

For stricter immutability, we do not recommend such compatibility.

As a workaround, you could also do it this way:

const f = (element: HTMLElement) => {};

const el: Immutable<HTMLElement> = document.body;

f(el as HTMLElement);