A pattern for protected class properties in ECMAScript

protected.js

"use strict";

class A {
	#writableSide;
	#writableSideInit = false;
	constructor(x) {
		this.#writableSide = {a: x};
	}
	initWritableSide() {
		if(this.#writableSideInit) throw new Error('writableSide already initialized');
		this.#writableSideInit = true;
		return this.#writableSide;
	}
}

class B extends A {
	#writableSide;
	constructor(x) {
		super(x);
		this.#writableSide = this.initWritableSide();
	}
	log() {
		console.log(this.#writableSide);
	}
}

new B(1).log();

protected1.js

"use strict";

class A {
	#writableSide;
	#hasWritableSide;
	constructor(x) {
		this.#writableSide = this.#hasWritableSide = {a: x};
	}
	get writableSide() {
		const writableSide = this.#hasWritableSide;
		this.#hasWritableSide = null;
		return writableSide;
	}
}

class B extends A {
	#writableSide;
	constructor(x) {
		super(x);
		this.#writableSide = this.writableSide;
	}
	log() {
		console.log(this.#writableSide);
	}
}

new B(1).log();

protected2.js

"use strict";

class A {
	#writableSide;
	#hasWritableSide;
	receiveWritableSide;
	constructor(x) {
		this.#writableSide = {a: x};
		this.receiveWritableSide(this.#writableSide);
	}
}

class B extends A {
	#writableSide;
	receiveWritableSide(val) {
		// for some reason this won't take until the super() call finishes, ahhhhhhh
		// "Cannot write private member #writableSide to an object whose class did not declare it"
		this.#writableSide = this.writableSide;
	}
	constructor(x) {
		super(x);
	}
	log() {
		console.log(this.#writableSide);
	}
}

new B(1).log();

protected3.js

"use strict";

class A {
	#writableSide = null;
	constructor(x) {
	}
	makeWritableSide() {
    if(this.#writableSide) throw new Error('Cannot re-initialize');
		this.#writableSide = {a: x};
		return this.#writableSide;
	}
}

class B extends A {
	#writableSide;
	constructor(x) {
		super(x);
		this.#writableSide = this.makeWritableSide();
	}
	log() {
		console.log(this.#writableSide);
	}
}

new B(1).log();

protected4.js

"use strict";

class A {
	#writableSide;
	constructor(x, passSecrets) {
		this.#writableSide = {a: x};
		passSecrets(this.#writableSide);
	}
}

class B extends A {
	#writableSide;
	constructor(x) {
		let v;
		super(x, (_v)=>{ v = _v; });
		this.#writableSide = vv;
	}
	log() {
		console.log(this.#writableSide);
	}
}

new B(1).log();