Javascript form validation without nested if-else while showing only one error

Question

I am trying to validate a form and want to check whether fields are empty. And if more than one fields are empty it will show error for only the first one of them.

So I was suggested to use nested if else for that. But that is creating an arrow shape code which I don't like.

Any way to make it a function with few lines?

var firstName = document.getElementByI("firstName").value;
var lastName = document.getElementByI("lastName").value;
var email = document.getElementByI("email").value;


if(firstname!=""){
    if(lastname!=""){
        if(email!=""){
            return "";
        }
        else
        {   document.getElementByI("email").focus();
            return "email is required";
        }
    }
    else
    {
        document.getElementByI("lastName").focus();
        return "lastname is required";
    }
}
else{
    document.getElementByI("firstName").focus();
    return "firstname is required";
}