<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Page title</title>
</head>
<body>
<button class="btn">прибавить</button>
<p class="p"></p>
</body>
<script>
let btn = document.querySelector('.btn')
let p = document.querySelector('.p')
let i = 0
btn.onclick = function () {
i++
p.innerHTML = "результат: " + i
}
</script>
</html>