onclickvar sayHello = function(){
console.log("hello");
};
var btn = document.createElement('input');
btn.type = "button";
btn.className = "btn";
btn.value = "Negociate";
btn.onclick = sayHello;
document.getElementById('mainPage').appendChild(btn);<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>BookInc</title>
<link rel="stylesheet" href="style.css">
</head>
<body style="background-color:white;">
<div id= "mainPage">
</div>
<script src="test.js"></script>
</body>
</html>btn.onclickbtn.onclick = () => console.log("hello);