lis[i].addEventListener('click', function() { lis[i].classList.add("lista"); erro nesta linha console.log(this.textContent); });
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<link rel="stylesheet" href="css/index.css">
<title>Document</title>
</head>
<body>
<ul>
<li>A</li>
<li>B</li>
<li>C</li>
</ul>
<script>
var lis = document.querySelectorAll('li');
for(var i = 0; i < lis.length; i++) {
lis[i].addEventListener('click', function() {// ela deseja exibir o conteúdo da li no clique
lis[i].classList.add("lista");
console.log(this.textContent);
});
}
</script>
</body>
</html>