svelte/test/runtime/samples/observable-auto-subscribe/_config.js

const subscribers = [];

let value = 'initial';

const observable = {
	subscribe: fn => {
		subscribers.push(fn);

		fn(value);

		return {
			unsubscribe: () => {
				const i = subscribers.indexOf(fn);
				subscribers.splice(i, 1);
			}
		};
	}
};

export default {
	props: {
		observable,
		visible: false
	},

	html: '',

	async test({ assert, component, target }) {
		assert.equal(subscribers.length, 0);

		component.visible = true;

		assert.equal(subscribers.length, 1);
		assert.htmlEqual(target.innerHTML, `
			<p>value: initial</p>
		`);

		value = 42;
		await subscribers.forEach(fn => fn(value));

		assert.htmlEqual(target.innerHTML, `
			<p>value: 42</p>
		`);

		component.visible = false;

		assert.equal(subscribers.length, 0);
	}
};