Unable to mark array methods as readonly

[bradzacher]

TypeScript Version: 3.7.5

Search Terms: array, methods, readonly

Code

class Foo {
    x(): void { }
    notReadonly = 1;
}

function foo1(arg: Readonly<Foo>) {
    arg.x = () => { };   // error ✅
    arg.notReadonly = 2; // error ✅
}

function foo2(arg: readonly string[]) {
    arg.length = 1;          // error ✅
    arg.forEach = () => { }; // no error ❌ - #22315
}
function foo3(arg: Readonly<ReadonlyArray<string[]>>) {
    arg.length = 1;          // error ✅
    arg.forEach = () => { }; // no error ❌
}

interface MyArray<T> extends Array<T> {
}

function foo4(arg: MyArray<string>) {
    arg.length = 1;          // no error ✅
    arg.forEach = () => { }; // no error ✅
}
function foo5(arg: Readonly<MyArray<string>>) {
    arg.length = 1;          // error ✅
    arg.forEach = () => { }; // error ✅
}

Expected behavior:
ReadonlyArray<T> and readonly T[] methods should be immutable by default, or should be able to be marked as immutable via Readonly<T>.

Actual behavior:
The typechecker has special handling under the hood which detects an array inside of Readonly<T> converts it to the ReadonlyArray type.

I.e.

Readonly<string[]> === readonly string[]               ✅
Readonly<readonly string[]> === readonly string[]      ❌
Readonly<Array<string>> === readonly string[]          ✅
Readonly<ReadonlyArray<string>> === readonly string[]  ❌

More Info:

• Methods are not readonly-by-default, and there's no direct way to declare a readonly method (Suggestion: readonly method #22315).
  • Classes/objects can, however, have their methods marked as readonly via Readonly<Class>.
• If you extend the array type (interface MyArray<T> extends Array<T> {}), then it works as expected, because the typechecker no longer thinks the type is an array type (checker.isArrayType(type) === false).
• I ran into this whilst building a rule which asserts that all function arguments are readonly (Rule proposal: Enforce read-only typescript-eslint/typescript-eslint#514).

Playground Link:
https://www.typescriptlang.org/play/?ssl=1&ssc=1&pln=30&pc=2#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

Related Issues:
#22315

[RyanCavanaugh]

Duplicate #35313

[bradzacher]

I don't think it's a duplicate, from my understanding of that issue.

This is specifically about being able to mark methods on the array type as readonly via Readonly<T>, so that you cannot reassign the methods.

#35313 is about making Readonly<T> remove methods from object types.

I don't want to remove anything - I just want to be add the readonly modifier to methods via Readonly<T>, which works fine for everything except array types.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.