Derived private property calculated after constructor

[shadow-identity]

Describe the bug

I was trying to instantiate private properties in my class, and to use some of them in the constructor. But unfortunately, the derived properties were null, even though they had all the necessary data to be valid.

This happens only with private derived properties. Non-private derived properties are initialized before the constructor and works fine. $state private properties are also working fine.

Reproduction

https://svelte.dev/playground/61fd75bbaa534daf8fbcef02580dbe9a?version=5.19.10

In this example, you will have console.log output from the constructor before one from derived.by

class Foo {
	#state = $state('state')
	#derived = $derived('derived from ' + this.#state); // no matter what is inside, it will be null in constructor
	#derivedBy = $derived.by(() => {
		console.log('derivedBy'); 
		return 'derived from ' + this.#derived  // no matter what it returns, it will be null in constructor
	});
	
	initial
	
	constructor() {
		this.initial = [this.#state, this.#derived, this.#derivedBy]
		console.log(this.initial) // ['state', null, null]
	}
	
	get derived() {return this.#derived}
	get derivedBy() {return this.#derivedBy}
	get state() {return this.#state}		
}

const foo = new Foo()

System Info

Just svelte 5.19.10, see repl.
Reproducible on Firefox 136 and Chrome 133, Safari 18.3

Severity

annoyance

[paoloricciuti]

Yeah i think we should fix this...in the meantime if you want as a workaround you can pull that logic in a separate method from the constructor to make it work

class Foo {
		#state = $state('state')
		#derived = $derived('derived from ' + this.#state);
	  #derivedBy = $derived.by(() => {
			console.log('derivedBy'); 
			return 'derived from ' + this.#derived 
		});
		
		initial
		
		constructor() {
			this.log();
		}

		log(){
			this.initial = [this.#state, this.#derived, this.#derivedBy]
			console.log(this.initial)
		}
		
		get derived() {return this.#derived}
		get derivedBy() {return this.#derivedBy}
		get state() {return this.#state}		
	}