Function prop

packages/svelte/src/internal/client/reactivity/props.js:279–436

(props, key, flags, fallback)

 * @returns {(() => V | ((arg: V) => V) | ((arg: V, mutation: boolean) => V))}
 */
export function prop(props, key, flags, fallback) {
	var runes = !legacy_mode_flag || (flags & PROPS_IS_RUNES) !== 0;
	var bindable = (flags & PROPS_IS_BINDABLE) !== 0;
	var lazy = (flags & PROPS_IS_LAZY_INITIAL) !== 0;

	var fallback_value = /** @type {V} */ (fallback);
	var fallback_dirty = true;
	var fallback_signal = /** @type {Derived<V> | undefined} */ (undefined);

	var get_fallback = () => {
		if (lazy && runes) {
			fallback_signal ??= derived(/** @type {() => V} */ (fallback));
			return get(fallback_signal);
		}

		if (fallback_dirty) {
			fallback_dirty = false;

			fallback_value = lazy
				? untrack(/** @type {() => V} */ (fallback))
				: /** @type {V} */ (fallback);
		}

		return fallback_value;
	};

	/** @type {((v: V) => void) | undefined} */
	let setter;

	if (bindable) {
		// Can be the case when someone does `mount(Component, props)` with `let props = $state({...})`
		// or `createClassComponent(Component, props)`
		var is_entry_props = STATE_SYMBOL in props || LEGACY_PROPS in props;

		setter =
			get_descriptor(props, key)?.set ??
			(is_entry_props && key in props ? (v) => (props[key] = v) : undefined);
	}

	/** @type {V} */
	var initial_value;
	var is_store_sub = false;

	if (bindable) {
		[initial_value, is_store_sub] = capture_store_binding(() => /** @type {V} */ (props[key]));
	} else {
		initial_value = /** @type {V} */ (props[key]);
	}

	if (initial_value === undefined && fallback !== undefined) {
		initial_value = get_fallback();

		if (setter) {
			if (runes) e.props_invalid_value(key);
			setter(initial_value);
		}
	}