careful_attack.js

const privateKey = Symbol('secret')

class SymbolPrivate {
	constructor() {
		this[privateKey] = 'SECRET'
		this.publicKey = 42
	}
	get privateKey() {
		return this[privateKey]
	}
	publicFunction() {
		return this.publicKey
	}
}

const instance = new SymbolPrivate()

const proxy = new Proxy(instance, {
	get: function (target, key) {
		Object.getOwnPropertySymbols(target).forEach(symbol => console.log(instance[symbol]))
		return target[key]
	}
})

SymbolPrivate.prototype.publicFunction.call(proxy)