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
}

ソースファイルもテストも指定されていません。まず issue 内のクラスメソッドと readonly プロパティの例を比較し、次に TypeScript がメソッドへの代入をチェックしている箇所を追跡してください。readonly-method の動作が決定され、提案されている breaking change に対する互換性対応が行われ、代入の拒否をカバーするテストが追加されれば完了です。