Tutorials on Lifecycle Method

As the most starred open source, JavaScript library/framework repository on GitHub, Vue.js has become a top three front-end technology alongside React.js and Angular in terms of popularity, usage, ecosystem activity and developer satisfaction. Compared to React.js and Angular, Vue.js is incrementally adoptable and provides a declarative API that resonates with AngularJS developers. Evan You , the author of Vue.js, explained the original concept of Vue.js as extracting the best parts of AngularJS, such as directives (i.e., v-if and v-show ), and building a lightweight, flexible alternative. Building large Vue.js applications requires composing components together with state management (i.e., Vuex ) and routing (i.e., Vue Router ) libraries. In September 2020, the Vue.js team officially released Vue 3, which welcomed a number of improvements and new features: Particularly, the Composition API of Vue 3 has been met with controversy due to its resemblance to React hooks and its philosophical divergence from the Options API , which emphasizes separation of concerns by defining component logic within specific options ( data , props , computed , methods , etc.). For those who prefer the Options API, unlike other major version upgrades that introduce incompatible changes, the Composition API will not break existing Vue components using Options API (" purely additive "). Although it offers similar logic composition capabilities as React hooks, the Composition API leverages Vue's reactivity system to circumvent several of React hooks' issues.

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 .

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 :

In Svelte, a component's afterUpdate lifecycle method is called after the component is updated as a result of a state change. When the value of any of a component's props and variables changes, the component's beforeUpdate lifecycle method is executed prior to any DOM updates being made in response to these changes. Once the beforeUpdate lifecycle method is finished running, the DOM is updated to reflect the data changes. After completing this update, the afterUpdate lifecycle method is executed. Being able to schedule a callback to run at this phase of a component's lifecycle ensures that the DOM is completely updated and synced with any new state and prop values. Here's a template of how the afterUpdate lifecycle method is used inside of a component: When using the afterUpdate lifecycle method, remember that it runs on every state and prop change. For components that experience many state and prop changes, it can be quite cumbersome to track which variables/props have changed in order to then execute the appropriate code. This is unlike React's componentDidUpdate lifecycle method , which provides the previous props and state as arguments.

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: