LWCDemo.html

<template>
    <div class="slds-text-heading_large slds-text-align_center">
        {greeting}
    </div>
</template>  

LWCDemo.js

import { LightningElement, track } from 'lwc';  
  
export default class LWCDemo extends LightningElement {  
  
    @track greeting;  
  
    connectedCallback() {
        this.greeting = 'Hello World!';
    }
}