Web part customization for SharePoint, button Onclick functionality not working in typescript

[lavanthilak]

Web part customization for SharePoint, button Onclick functionality not working in typescript. We customized the web part by using NPM and typescript. In typescript, the onclick event is not working when we assigned the function call for that button. Can you please provide the solution for this?

public openNewPage():void  {
let site = this.context.pageContext.site.absoluteUrl;
console.log("Site: "+site);

const webPart: CreatePageWebPart = this;
window.onload= function (){
this.document.querySelector('#btnCopy').addEventListener('click', () => {
  var headtext =  document.getElementById("pageName");
  alert(headtext);
  
  });
}}