Common error message in JavaScript

I can see a couple of problems in your code.

• Mismatch between errorMsg()'s expected arguments and how it is called
• Syntax error in second alert()
• Bad expression inside if statement

Mismatch between errorMsg()'s expected arguments and how it is called

Your errorMsg() function expects three arguments, but you only pass it two at a time:

errorMsg: function(title,FirstNameValue,LastNameValue) {
    ....
}

... and then ....

.errorMsg('FirstName',FirstNameValue);
.errorMsg('FirstName',LastNameValue);

If you really want to use both values inside errorMsg(), you need to pass them both every time, in the same order the function expects them:

PatientModel.errorMsg('FirstName',FirstNameValue,LastNameValue);
PatientModel.errorMsg('LastName',FirstNameValue,LastNameValue);
// This is weird code, but it'll work

Syntax error in second alert()

This is simple enough to fix, and could have been just a typo.

alert(+title 'is missing');
      ^      ^_There's something missing here.
      |_This will only try to convert title to a number

What you want is this:

alert(title + 'is missing');

Bad expression inside if statement

if(FirstNameValue === undefined || FirstNameValue === ' ' && LastNameValue === undefined || LastNameValue = ' ') {

This won't work as you expect, because && has greater precedence than ||, meaning the expression will be evaluated as such:

if (
     FirstNameValue === undefined
 || (FirstNameValue === ' ' && LastNameValue === undefined)
 ||  LastNameValue = ' '
) {

You would need parenthesis to fix the precedence:

if( (FirstNameValue === undefined || FirstNameValue === ' ') && (LastNameValue === undefined || LastNameValue = ' ') ) {

This is irrelevant, actually, because the expression can be simplified like this:

// If these values are taken from a form input, they are guaranteed to be strings.
if(FirstNameValue.length === 0 && LastNameValue.length === 0) {

Or even better, like this:

// Uses regular expressions to checks if string is not whitespace only
var WHITESPACE = /^\s*$/;
if( WHITESPACE.test(FirstNameValue) && WHITESPACE.test(FirstNameValue)){

How I would fix your code

This would be an incomplete answer if I didn't provide a correct version of your code, so here it goes. Notice that I separate filling-in of information and its validation in two steps.

PatientModel.js :

validate: function(patient){
    var WHITESPACE = /^\s*$/;
    var errors = [];

    if( WHITESPACE.test(patient.FirstName) ){
        // No first name
        if( WHITESPACE.test(patient.LastName) ){
            // No last name either
            errors.push('FirstName and LastName are missing');
        }
        else {
            // Only first name missing
            errors.push('FirstName is missing');
        }
    }
    else if( WHITESPACE.test( patient.LastName) ){
        // Only last name missing
        errors.push('LastName is missing');
    }

    // Returns array of errors
    return errors;
}



Your other code:

AddPatient = {};
AddPatient.Firstname = FirstNameValue; 
AddPatient.LastName = LastNameValue;

errors = PatientModel.validate(AddPatient);
if( errors.length != 0 ){
    alert('You have the following errors:\n' + errors.join('\n'));
}

Edit: a different, perhaps better, approach. The only difference is we now write validate() as a method of a Patient object:

>>> PatientModel.js:

var WHITESPACE = /^\s*$/;

// Creates a new empty Patient
function Patient(){
    this.FirstName = '';
    this.LastName = '';
}

// Adds a validate() method to all Patient instances
Patient.prototype.validate: function(){
    var errors = [];

    if( WHITESPACE.test(this.FirstName) ){
        // No first name
        if( WHITESPACE.test(this.LastName) ){
            // No last name either
            errors.push('FirstName and LastName are missing');
        }
        else {
            // Only first name missing
            errors.push('FirstName is missing');
        }
    }
    else if( WHITESPACE.test( thisLastName) ){
        // Only last name missing
        errors.push('LastName is missing');
    }

    // Returns array of errors
    return errors;
}




>>> Your other code :

patient = new Patient();
patient.FirstName = FirstNameValue;
patient.LastName = LastNameValue;

errors = patient.validate();
if( errors.length != 0 ){
    alert('You have the following errors:\n' + errors.join('\n'));
}