Svelte - on:event listener removal

Question

I'm wondering whether assigning undefined to an element's event, ie on:mousemove prevents a memory leak the same as removeEventListener does, or at least should not be a concern long term. I've checked with getEventListeners on chrome, and it's there with the on:mousemove approach, but I'm not sure whether I should worry about this and use the more verobse approach with custom actions.

I have a Dropdown inside a Container. On moving the mouse over the container, I want to change the Dropdown's position.

My initial approach was writing a custom use:containerMouseMove action, that took Dropdown's visible as a dependency, and removed the event listener from the container, when the Dropdown became invisible.

Dropdown.svelte:

        use:mousemoveContainer={{ container, destroyOn: !visible }}
        on:mousemove_container={(e) => {
            if (mouseTrack) {
                [x, y] = calcCoordinates(e, container);
            }
        }}

Action definition:

type Deps = { container: HTMLElement; destroyOn: boolean };

export const mousemoveContainer = (node: HTMLDivElement, deps: Deps) => {
    const handleMouseMove = (e: MouseEvent) => {
        node.dispatchEvent(new CustomEvent('mousemove_container', { detail: e }));
    };
    return {
        update(deps: Deps) {
            if (!deps.destroyOn) {
                deps.container.addEventListener('mousemove', handleMouseMove);
            }
            if (deps.destroyOn) {
                deps.container.removeEventListener('mousemove', handleMouseMove);
            }
        }
    };
};

Then I learned about export const function as a way to communicate between parent and child, and it simplifies the code. But I'm not sure if there's not a memory leak right now.

    on:mousemove={dropdown.getVisible() ? dropdown.onContainerMouseMove : undefined}

onContainerMouseMove is the callback inside on:mousemove_container .

Answer 1

on:event listeners are removed automatically when the component is unmounted.

Within actions, one should return an object with a destroy() function and unsubscribe from events there. The function will be called when the element with the action on it is removed.