Type assertion of readonly array property doesn't work only with a class type (static property).

[miguel-leon]

🔎 Search Terms

type assertion readonly array

🕗 Version & Regression Information

Latest as of today.

⏯ Playground Link

https://www.typescriptlang.org/play/?#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

💻 Code

interface NormalArray {
	arr: string[];
}


/* --- object type --- */

declare const a: { arr: readonly string[] }; // also works with an interface here.
(a as NormalArray); // works 🎉


/* --- namespace --- */

declare namespace N {
	export const arr: readonly string[];
}
(N as NormalArray); // works 🎉

declare const n: typeof N;
(n as NormalArray); // works 🎉


/* --- */

declare class Foo {
	static arr: readonly string[];
	arr: readonly string[];
}

/* --- instance --- */

declare const i: Foo;
(i as NormalArray); // works 🎉

/* --- class --- */

(Foo as NormalArray); // doesn't work ❌

declare const c: typeof Foo;
(c as NormalArray); // doesn't work ❌

🙁 Actual behavior

The type assertion works with everything except on static properties.

🙂 Expected behavior

Works with everything?

Additional information about the issue

Is there a special reason for this not to work with static properties? Or should this work in all cases? Or should this not work in any case?

[jcalz]

Very roughly speaking TS allows you to type assert when you're either upcasting/widening or downcasting/narrowing, but not when you're arbitrarily, uh, sidecasting. All of the examples here except for the class constructor seem to be downcasting (since readonly string[] is wider than string[]) so they succeed. But class constructors have prototype, so while arr of typeof Foo is wider than the arr of NormalArray, the prototype is narrower. That's sidecasting and TS complains. If you add other properties to the other cases you'll see the same error (see playground link). So this seems to be working as intended, not a bug.

[miguel-leon]

@jcalz do you also have the explanation of why the error is:

Types of property 'arr' are incompatible.
    The type 'readonly string[]' is 'readonly' and cannot be assigned to the mutable type 'string[]'.

rather than mentioning the two different casting directions which according to your explanation is the cause?

Is this misleading error also intended?