Bug Report

🔎 Search Terms

Typescript 5, decorator, emit, __runInitializers, super, constructor

🕗 Version & Regression Information

Version v5.1.0-dev.20230310 (nightly)

• This changed between versions v4.9.5 and v5.0.0-rc
• I was unable to test this on prior versions because the new decorator spec was not supported

⏯ Playground Link

Playground link with relevant code

💻 Code

class Foo {}

class Bar extends Foo {
	public constructor() {
		console.log('Entering constructor.');
		super();
		console.log('Exiting constructor.');
	}

	@loggedMethod
	public greet(): void {
		console.log('Hello!');
	}
}

function loggedMethod(method: any, _context: any): any {
	return function (this: any) {
		console.log('Entering method.');
		method.call(this);
		console.log('Exiting method.');
	};
}

new Bar(); /* JS Error:
Must call super constructor in derived class before
accessing 'this' or returning from derived constructor */

🙁 Actual behavior

The emitted output includes the following (abbreviated for clarity; see entire output in playground):

// ...
class Bar extends Foo {
	// ...
	constructor() {
		__runInitializers(this, _instanceExtraInitializers); // <-- here!
		console.log('Entering constructor.');
		super();
		console.log('Exiting constructor.');
	}
	greet() {
		console.log('Hello!');
	}
}

__runInitializers(this, _instanceExtraInitializers); was emitted before the call to super(); in the constructor. This is a problem because this cannot be referenced before super() is called. The following error is thrown:

Must call super constructor in derived class before accessing 'this' or returning from derived constructor.

🙂 Expected behavior

I expect the call to __runInitializers to be emitted after super:

constructor() {
	console.log('Entering constructor.');
	super();
	__runInitializers(this, _instanceExtraInitializers);
	console.log('Exiting constructor.');
}

🔄 Workaround

Add a private field. Private fields are initialized after super(), so the location where __runInitializers is emitted isn’t a problem.

Source:

class Bar extends Foo {
	readonly #HACK = 0;

	public constructor() {
		console.log('Entering constructor.');
		super();
		console.log('Exiting constructor.');
	}

	// ...
}

Output:

// ...
class Bar extends Foo {
	// ...
	#HACK = (__runInitializers(this, _instanceExtraInitializers), 0);
	constructor() {
		console.log('Entering constructor.');
		super();
		console.log('Exiting constructor.');
	}
	// ...
}

(Note/Reminder: Even though the field is written above the constructor, it isn’t executed until after the super() call.)