Lifecycle hooks in LWC

There are 3 phase of LWC component 

1. Mounting 
A. constructor, B. connnectedCallback C. render D. renderedCallback

2. UnMounting 
A. disconnectedcallback

3. Error 
A.errorcallback

Note - render is not lifecycle hook, it is protected method of Lightning element class.

Mounting Phase

LWC Creation and Render Life cycle

Constructor Method
·       This method called when component is instantiated and It flows from parent to child component.
·       Need to call Super() inside constructor method
·       Can’t access any component properties or child component because it’s not ready yet.
·       Host element can be accessed through “this. template” inside constructor method.
·       Don’t add any attributes to host inside constructor

C constructor(){

        super(); // This is mandatory

        console.log('parent component constructor called ');

        //var elements = this.template.querySelector('.demo'); // We cant do this coz component

is not loaded yet. however we can add event listener to whole component

       // console.log('elements..',elements);

    }

connectedCallBack Method
·       This method called when component is inserted into DOM and It flows from parent to child component.
·       Can’t access any  child component property because it’s not exists yet.
·       Host element can be accessed through “this. template” inside this method.
·       Use connectedCallBack to perform initialization task such as fetch data, set up caches,listen to publish-subscribe events or call apex method.

Dont use this method to change the state of component properties loading values or setting properties. Use getters and setters instead.

connectedCallback(){

        console.log('parent component connectedCallback called ');

       

 }

renderedCallback Method
·       This method called when component is rendered and It flows from child to parent component.
·       It can fired more than once, any changes made to component invokes this method again and again which impact performance
·      When component re-renders all expression used in the template are reevaluated 
·       Don’t use renderedCallback to change the state or update property of a component

Do not use rerenderedCallback to update wire adapter configuration object property, it will result infinite loop.

renderedCallback(){

        console.log('parent component renderedCallback called ');

       

}

UNMounting Phase

disconnectedCallback Method
·       This method called when component is removed from DOM
·       It flows from Parent to child and it is specific Lightning Web component.

disconnectedCallback(){

        console.log('parent component disconnectedCallback called');

    }

Uses

1. disconnectedCallback can be used for performance improvement and memory leakage issue.

for example, If event listener or set Interval of window property set,

then It will be running behind the scene, we can remove (removeEventlistner or clearInterval)

in this method.

Error Phase

disconnectedCallback Method

This method called when  descended component throw an error.

errorCallBack have 2 arrgurment error object and stack argument as String

errorCallback(error,stack){

        console.log('stack',stack);

        console.log('error',error);

        console.log('error message',error.message);

 }

render Method

• render method is a protected method of Lighting Element Class.
• This method tells the lightning engine which template to load based on some condition, The return type of this method is reference of template.
• We dont have to keep writing all html code in html file, we can segregate to different html file. By using render method we can load. 

 Importing html file.

import secondHTML from './second.html';

import firstHTML from './first.html';

render(){

        return this.htmlFileName === 'secondHTML' ? :firstHTML;

       

    }