Class method decorators reset the first field.

[xiaoxiyao]

🔎 Search Terms

"__runInitializers", "decorator"

Possibly related issues：#54264

🕗 Version & Regression Information

• I was unable to test this on prior versions because non-experimental decorators are only available in 5.0+

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABAIzuAJgCjgJxgcxjAEMAbAWQFMoALOdALkWLAE8AaRCBKSgDyhMAwqWIBnMVVr0AIpW45iUXEJ78oASiYA3ODHSIA3gChEXBGKiIAttTroAcsVuIAvObC8BAOhK2A3KYeXlDexOjoAJJgMLBkMABelDiYoJCwCIiYtDBiTCysGkZBZjliANq20o7OlAC6bohllXb0TrZ13shEWGUagWYAvv3Gg8bGEKISiABC4pTFZtxgljgg0LiYRSZmpTS53kSxWwOIY0EADjhwvNCUBkdQW8VBY+eT4mKIAGJwcIjqShgdBfOZiBY7FDEHAAfiYqyI+ECQTg2mSeHQC0ez0hZjEIAuyUOMSeI12TX2Yi60MaAHIaJRSKQ4LTTuczAABVAYS4gZCkGAQRDoOAAZTgVX2YHwOPGQ1GEwscFIlG8zJlYEoAHcfn8ttScP0gA

💻 Code

function bound(originalMethod: any, context: ClassMethodDecoratorContext): void {
  const methodName = context.name;
  context.addInitializer(function (this: any) {
    this[methodName] = this[methodName].bind(this);
  });
}

class Base {
  constructor() {
    this.init();
  }

  protected init() {

  }
}

class Foo extends Base {
  bar?: string;

  override init() {
    super.init();
    this.bar = 'hello';
  }

  @bound
  public doSomething() {

  }
}
console.log(new Foo().bar);

🙁 Actual behavior

Prints undefined

🙂 Expected behavior

Prints hello

Additional information about the issue

The generated JS is as follows:

constructor() {
      super(...arguments);
      this.bar = __runInitializers(this, _instanceExtraInitializers);
}

Here, it always assigns to the first field, which should be a bug.

[MartinJohns]

This is working as intended and independent of using decorators. You have the same result with this code:

class Base {
  constructor() {
    this.init();
  }

  protected init() {

  }
}

class Foo extends Base {
  bar?: string;

  override init() {
    super.init();
    this.bar = 'hello';
  }
}
console.log(new Foo().bar);

This results in "hello" being printed when useDefineForClassFields is turned off, and undefined when it's turned on. Using the decorators requires useDefineForClassFields to be turned on.