action() — svelte Function Reference

Source Code

packages/svelte/src/internal/client/dom/elements/actions.js lines 13–43

export function action(dom, action, get_value) {
	effect(() => {
		var payload = untrack(() => action(dom, get_value?.()) || {});

		if (get_value && payload?.update) {
			var inited = false;
			/** @type {P} */
			var prev = /** @type {any} */ ({}); // initialize with something so it's never equal on first run

			render_effect(() => {
				var value = get_value();

				// Action's update method is coarse-grained, i.e. when anything in the passed value changes, update.
				// This works in legacy mode because of mutable_source being updated as a whole, but when using $state
				// together with actions and mutation, it wouldn't notice the change without a deep read.
				deep_read_state(value);

				if (inited && safe_not_equal(prev, value)) {
					prev = value;
					/** @type {Function} */ (payload.update)(value);
				}
			});

			inited = true;
		}

		if (payload?.destroy) {
			return () => /** @type {Function} */ (payload.destroy)();
		}
	});
}