Property initializers output for non-initialized types

Bug Report

This is technically not a bug, given ES Spec, however, I'm filing an issue in case an consideration needs to be made on how to handle with regard to documentation, etc, and/or to provide a solution for people facing the same.

Issue Summary

• ESNext spec seems to call for defining all class fields. This also affects those which are optional or specified with a boom.
• Spec also dictates that property initializers are called after the super call. (https://github.com/tc39/proposal-class-fields)

This results in any properties specified in the inherited class that are assigned during the base class constructor to be overwritten, even if they do not have initializer values specified in the inherited class.

The behaviour which causes the error can be seen below:

class A {
  a?: string
}

Output is:

// target = ESNext
class A {
  a; // Outputs a statement without initialized value, which is treated as a = undefined;
}

// target = ES2020
class A {
}

Here is a simplified version of how this affected me

abstract class Base<T extends Record<string, any>> {
  // Single constructor for all derivatives
  constructor(o: Omit<T, typeof Base>) {
    Object.assign(this, o);
  }
}

class A extends Base<A> {
  myProp?: string
  myProp2!: string
}

const a = new A({ myProp2: 'hello' });

// ESNext: a = { myProp2: undefined, myProp: undefined }
// ES2020: a = { myProp2: 'hello' }

The above produces:

// TypeScript target=ESNext
class A extends Base {
  myProp;
  myProp2; // Note the boomed property still gets output here, so it is initialized after the super call - which maybe surprising behaviour
}

// ES2020 / Babel
class A extends Base {
}

🔎 Search Terms

• property initializers

🕗 Version & Regression Information

TS 4.3.5

Solution

For those facing this issue, simply change property declarations to ambient.

ie:

class A extends Base<A> {
  declare myProp?: string
  declare myProp2: string
}

issue にある最小限のクラス例から始め、optional プロパティと definite-assignment プロパティを含めて、ESNext ターゲットと ES2020 ターゲットで出力される結果を比較します。意図された結果がコンパイラーの変更なのかドキュメントなのかを判断し、そのうえで、選択した動作によってベースコンストラクターが割り当てた値が上書きされないことを確認します。