Chapter 24: Render Lifecycle Hook in Lightning Web Component -

Render is mainly used to conditionally render a template. It defines the business logic to decide which template (HTML file) to use.

Enter the render() method

In LWC this method doesn’t do the actual rendering, but determines what template to use to render the component.

There are a few simple points:

We need to import the template into the JavaScript file
The call to return must return the imported variable

Use Case: We have a salesTemplate and on click of which we are able to redirect to service template.

In this use case we have following file structure:

salesTemplate.html
serviceTemplate.html
salesTemplate.js

If you observe, we have two templates and we are conditionally rendering them based on the component’s state.

Code Snippet.

salesTemplate.html

<template>
<lightning-card title="Example of render() lifecycle hook">
        This is the Sales Template
        <br/> <br/>
        <lightning-button 
        label="Go to the service template" 
        onclick={onchange}>
        </lightning-button>
 </lightning-card>
</template>

salesTemplate.js

import { LightningElement } from 'lwc';
import firsttemplate from './salesTemplate.html';
import secondtemplate from './serviceTemplate.html';

export default class SalesTemplate extends LightningElement {
  templatename = 'Salestemplate';
        constructor(){
            super();
            console.log("Inside Constructor- salestemplate");
               }
        connectedCallback(){
            console.log("Inside ConnectedCallback -salestemplate");
               }
               
    onchange(){
        if (this.templatename === 'Salestemplate' ) {
            this.templatename = 'Servicetemplate';
        }
        else{
            this.templatename = 'Salestemplate';
        }

    }

    render(){
        console.log("Inside render")
        if (this.templatename === "Salestemplate") {
           console.log("Inside render -salestemplate")
            return firsttemplate;
            
        }
        else{
            console.log("Inside render -servicetemplate")
            return secondtemplate;
        }
    }
    renderedCallback(){
        console.log("renderedcallback");

    }
   
}

serviceTemplate.html

<template>
    This is service template;
</template>

Demo

Understanding Code Line by Line:JS

import firsttemplate from './salesTemplate.html';
import secondtemplate from './serviceTemplate.html';

I have imported two templates in the JavaScript. template references are firsttemplate and secondtemplate.

onchange(){
if (this.templatename === 'Salestemplate' ) {
this.templatename = 'Servicetemplate';
}
else{
this.templatename = 'Salestemplate';
}
}

onClick of button this onChange is executed. If template is SalesTemplate then it changes it to ServiceTemplate.

render(){
if (this.templatename === "Salestemplate") {
console.log("Inside render -salestemplate")
return firsttemplate;
}
else{
console.log("Inside render -servicetemplate")
return secondtemplate;
}
}

I have added condition in the render() method to return the correct template depending on the component’s state. The returned value from the render() method must be a template reference i.e. firsttemplate or secondtemplate, which is the imported default export from an HTML file.

I hope you enjoyed the chapter !

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Reference

Render Multiple Templates