helloWorld.js

import { LightningElement, track } from 'lwc';
export default class HelloWorld extends LightningElement {
//    @track greeting = 'World';
    @track firstName = '';
    @track lastName = '';
    @track showLastNameOnly = false;
    changeHandler(event) {
       // this.greeting = event.target.value;
        const fldValue = event.target.name;
        if(fldValue === 'first_name')
            this.firstName = event.target.value;
        else if(fldValue === 'last_name')
            this.lastName = event.target.value;
    }
    handleCheckBox(event){
        this.showLastNameOnly = event.target.checked;
    }
    get showLastNameOnly(){
        return this.showLastNameOnly;
    }
    get upperCaseName(){
        if(this.showLastNameOnly){
            return `${this.lastName}`.toUpperCase();
        }
        else{
            return `${this.firstName} ${this.lastName}`.toUpperCase();
        }
    }
}