<!DOCTYPE html>
<html>
<head>
<script src="script.js"></script>
</head>
<body>
<form>
<input type="emal" id="email" value="" placeholder="user">
<input type="password" id="contrasena" value="" placeholder="pass">
<button type="button" id="envia">Envia</button>
</form>
</body>
</html>
window.onload = function()
{
var envia;
var email;
var contrasena;
envia = document.getElementById('envia');
email = document.getElementById('email');
contrasena = document.getElementById('contrasena');
envia.onclick = function(){
var valor1 = email.value;
var valor2 = contrasena.value;
if(valor1 === "" || valor2 === "") {
alert("Error: los campos email o contraseña estan vacios.");
}
else{
alert("acceso Correcto");
location.href="direccion.html";
}
}
}
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<h1>Hola Mundo</h1>
</body>
</html>