In Svelte, a component's onDestroy lifecycle method is called before the component is removed from the DOM. For example, if you have a component that is wrapped within an {#if} block and it is currently rendered to the DOM, then when this block's conditional statement evaluates to false , the component will be unmounted from the DOM, at which point onDestroy will be called. Being able to schedule a callback to run at this phase of a component's lifecycle ensures that cleanup-related operations can be ran and previous application states can be resumed. Here's a template of how the onDestroy lifecycle method is used inside of a component: Compared to the function returned from onMount , there are specific reasons for using onDestroy :

Modern applications involve many DOM manipulations. Components (and elements) are constantly being removed, added, updated, etc. to deliver incredible user experiences and/or to optimize performance. Sometimes, you may need to execute code when a component is added (such as automatically focusing an input field when a form is loaded like the G-Mail login page) or when a component is removed (such as removing all event listeners associated with that element to prevent memory leaks). Frameworks/libraries such as React.js and Svelte provide component lifecycle methods that allow developers to predictably execute code at these very moments/situations. Having such access into each stage a component undergoes provides more control over how and when the component should be rendered. Commonly, functions can be scheduled to be invoked when these events occur in the lifecycle of a component instance: