Suggestion: readonly method

Problem:

A method (function written with the ES6 method syntax) can currently be replaced with another function by a simple assignment.

As an example consider the following snippet of code:

class Person {
    constructor(...) {...}
    displayName (): string {...}
}
const x: Person = new Person(...)
x.displayName = function(this: Person): string {...} // Assignment

Because the readonly modifier is not usable for methods, this is not possible to prevent this kind of assignments.

Proposal:

A method is always readonly.

The following codes are identical:

class Person {
    displayName (): string {...}
}

class Person {
    readonly displayName: (this: Person) => string = function () {...}
}

Compatibility:

This is a breaking change. However, method syntax is recent and mostly used in classes. Codes which assign a function to a method are certainly rare.

Temporary workaround:

Do not use method syntax in your classes and interfaces. Note hat this leads to very verbose codes.

If you use an interface, the verbosity is acceptable. However you get also strict variance.

interface PersonI {
    readonly displayName: (this: Person) => string
}

[feeddageek]

I’d like to add the the Readonly mapped type currently work on method event if the readonly keyword it self is not allowed.

class SomeClass {
    //public readonly SomeMethod() {} // 'readonly' modifier can only appear on a property declaration or index signature.
    public readonly SomeOtherMethod() {}
}
const someObject =  new SomeClass();
someObject.SomeOtherMethod = () => {}; // OK
const someOtherObject = new SomeClass() as Readonly<typeof SomeClass>
someOtherObject.SomeOtherMethod = () => {}; // Cannot assign to 'SomeOtherMethod ' because it is a constant or a read-only property.

A flag under the strict umbrella to make all method readonly by default and force the use of property function for “assignable method” would also be welcome.

[adrian-gierakowski]

I’d really like to see this happen as currently using classes with one or more methods as a function argument lead to a linter error when https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-readonly-parameter-types.md is enabled.

It’s also quite inconsistent that a class property can be made immutable but a method cannot