In Svelte, a component's onMount lifecycle method is called after the first time the component has been rendered to the DOM. For example, if you have a component that is wrapped within an {# if} block, then when this block's conditional statement is fulfilled, the component will be mounted to the DOM, at which point onMount will be called. Being able to schedule a callback to run at this phase of a component's lifecycle ensures that DOM elements within the component will be available for your code to access/manipulate. Here's a template of how the onMount lifecycle method is used inside of a component: Considered the most commonly used lifecycle method, onMount covers a wide diversity of use cases such as...

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 :