Conditional javascript input field validation

You could use the following:

var forms = {
  user: 0,
  email: 1
};

function whichForm() {
  var userForm = document.getElementById("user_number").value;
  var emailForm = document.getElementById("email").value;

  if (userForm && emailForm) {
    //user wrote in both forms, something is wrong
  } else if (!userForm && !emailForm) {
    //user didn't fill in any form
  } else {
    return userForm ? forms.user : forms.email;
  }
}

function empty(form) {
  if (form === forms.user) {
    // check if the user number form is empty
    var userForm = document.getElementById("user_number").value;
    if(userForm.trim() === "") {
      // possibly do more validation
      // return true or false based on whether you want to submit
    }
  } else if (form === forms.email) {
    // check if the email form is empty
    var emailForm = document.getElementById("email").value;
    if(emailForm.trim() === "") {
       // possibly do more validation
      // return true or false based on whether you want to submit
    }
  } else {
    // something is wrong, invalid parameter,
    // handle here
    return false
  }
}

function validate() {
  return empty(whichForm());
}

And change your form so that it calls return validate() inline or just validate as a submit handler.

Sounds like this would be enough?

I would personally not call the function empty since you want to return true to allow submission

function empty() {
  var x = document.getElementById("user_number").value,
      y = document.getElementById("email").value;
  x = x?x.trim()|| ""; // handle null and all blanks
  y = y?y.trim()|| "";

  if (x === "" && y === "") {
    alert("Please enter user number or email")
    return false;
  }
  // optional
  if (x && y) { // both entered
    alert("Please enter EITHER user number or email")
    return false;
  }

  if (x) return isValidUser(x); // each of these functions needs to return boolean
  if (y) return isValidEmail(y);

  // likely not going to happen
  return false;
}

You can test if both are empty

function empty() {
  var a = document.getElementById("user_number").value,
      b = document.getElementById("email").value;
  if ("" == a && "" == b) return MsgBox("Your user number or mibile is required.", "ERROR"), !1
};

Code do it in this way:

function empty() {
  var x = document.getElementById("user_number").value,
    y = document.getElementById("email").value;
  if (!x && !y) {
    alert('You should choose email address or number');
    return false;
  }
  return true;
}

Proposed solution: Check which of the two input fields is filled up:

var inputA = document.getElementById('A').value;
var inputB = document.getElementById('B').value;

if ((inputA !== "") || (inputA !== NaN) || (inputA !== undefined)) {
  //execute code for the user number
}
else if ((inputB !== "") || (inputB !== NaN) || (inputB !== undefined)) {
  //execute code for the email
}
else {
  //display an error saying none of the two fields were used
}

Recommendation: Most websites would only use 1 input because it looks a lot cleaner. And the place holder text can specify to the user what input options he should put in:

<input type="text" id="input1" placeholder="user number or email">

Proposed Solution: Check if the user input has a @ symbole:

var input1 = document.getElementById("input1").value;
var emailInput = input1.includes('@');//returns a boolean with a value of true
                                     //if '@' was found in the string input1
if (emailInput) {
  //execute the code for the email input
} else {
  //execute the code for the userID input
} 

Explanation:

I assumed that you wanted to use the same input field inside your <form ...> tag regardless if the user is using an email or an id number to log in. From that, what I saw as most logical is to just find something that is unique to one of those inputs, and base the logic of your code on whether that unique element existed in the input provided.

AKA since emails always have the @ symbol, verifying if this exists in the provided string or not should be enough to verify if the user used an email or id number to attempt to login.

Let me know if that helped :).