Tutorials on Beforeupdate

In Svelte, a component's beforeUpdate lifecycle method is called before the component is updated as a result of a state change. When the component receives new prop values or has its local state modified, the component's beforeUpdate lifecycle method is called before any updates to the DOM are made. Once the beforeUpdate lifecycle method finishes executing, the DOM will be updated with these data changes, which will allow the subsequent call to afterUpdate to have access to a completely synced DOM. Being able to schedule a callback to run at this phase of a component's lifecycle ensures that the value of certain DOM properties, such as a container element's scrollTop , can be cached prior to being updated. Then, these cached values can be used to revert those properties back to what they were originally. Here's a template of how the beforeUpdate lifecycle method is used inside of a component: Note : beforeUpdate is called before the component's initial onMount .

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: