svelte/test/runtime/samples/binding-select-multiple/_config.js

export default {
	get props() {
		return { selected: ['two', 'three'] };
	},

	html: `
		<select multiple>
			<option value="one">one</option>
			<option value="two">two</option>
			<option value="three">three</option>
		</select>

		<p>selected: two, three</p>
	`,

	async test({ assert, component, target, window }) {
		const select = target.querySelector('select');
		const options = [...target.querySelectorAll('option')];

		const change = new window.Event('change');

		options[1].selected = false;
		await select.dispatchEvent(change);

		assert.deepEqual(component.selected, ['three']);
		assert.htmlEqual(
			target.innerHTML,
			`
			<select multiple>
				<option value="one">one</option>
				<option value="two">two</option>
				<option value="three">three</option>
			</select>

			<p>selected: three</p>
		`
		);

		options[0].selected = true;
		await select.dispatchEvent(change);

		assert.deepEqual(component.selected, ['one', 'three']);
		assert.htmlEqual(
			target.innerHTML,
			`
			<select multiple>
				<option value="one">one</option>
				<option value="two">two</option>
				<option value="three">three</option>
			</select>

			<p>selected: one, three</p>
		`
		);

		component.selected = ['one', 'two'];

		assert.ok(options[0].selected);
		assert.ok(options[1].selected);
		assert.ok(!options[2].selected);

		assert.htmlEqual(
			target.innerHTML,
			`
			<select multiple>
				<option value="one">one</option>
				<option value="two">two</option>
				<option value="three">three</option>
			</select>

			<p>selected: one, two</p>
		`
		);
	}
};