How to find out if an element has a class?

You can use the (read-only) property classList, which returns a string with the different classes separated by spaces (as they would be in your HTML):

var item = document.getElementById("Nombre");
document.write( '<br>', item.classList );

<p id="Nombre" class="clase1 clase2 clase3" >
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It has some useful methods, such as contains( ):

var item = document.getElementById("Nombre");
var hasClase2 = item.classList.contains( 'clase2' );
document.write( '<br>', 'Tiene la clase "clase2": ', hasClase2 );

<p id="Nombre" class="clase1 clase2 clase3">
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List of supported methods:

add( String [, String] )

Add the indicated classes. If these classes exist in the element attribute they will be ignored.

remove( String [, String] )

Delete the indicated classes. Note: Removing a class that does not exist does NOT cause an error.

item(Number)

Returns the value of the class by index in the collection.

toggle( String [, force] )

When only one argument is present: Toggles the value of the class; eg if the class exists it removes it and returns false, if not it adds it and returns true.
When the second argument is present: If the second argument evaluates to true, the indicated class is added, and if it evaluates to false, it is removed.

contains( String )

Checks if the indicated class exists in the class attribute of the element.

replace(oldClass, newClass)

Replaces an existing class with a new one.

If what you want is to make a conditional based on whether or not an element has a class you can do this very quickly:

if (document.querySelector("#nombre.clase2")) {
  document.write("Esto solo se ejecuta si el elemento con id 'nombre' tiene la clase 'clase2'")
} else {
  document.write = "Se ejecuta si no la tiene"
}

<input id="nombre" class="clase2">

There are several ways to see the class of an element, even the previous answers are quite good, but if you want to expand the options when solving the problem, here are some (although they may be repeated with some other answers).

In Javascript with .classNameyou can see all the classes that an element has as a String.

.classNameyou could refer to a .classList.valuesince they both give a String and not an array as it would be on its .classListown.

var input = document.getElementById("Nombre");
// con .className obtienes todas las clases que pueda poseer un elemento
document.write(input.className);

<input id="Nombre" class="clase1 clase2 clase3" type="text" />
<br>
Clases que posee:

In a different way you can use .classListor .classList.valueas I explained before

Remember that it .classListonly returns an Array not a String , it only returns .classList.valuea String of all classes with this property.

var input = document.getElementById("Nombre"); document.write(input.classList)

<input id="Nombre" class="clase1 clase2 clase3" type="text" />
<br>
Clases que posee:

A different option would be to use .containsto verify with Booleans if it is true or not that it has the class that is being searched for .

This method returns its value when you combine it with another property (see last example)

var input = document.getElementById("Nombre"); document.write('Posee la clase "clase2":' + ' ' + input.classList.contains("clase2")); 

<input id="Nombre" class="clase1 clase2 clase3" type="text" />
<br>

Another option you have is to use a combination of properties mentioned above, for example:

To see how it works, change the class "clase2" which is the one you are looking for

This method does not work .classNamebecause this is not an Array

var input = document.getElementById("Nombre");
if (input.classList.contains("clase2")){  
document.write(input.classList + " " + "<br>" + "<b>Si posee la clase2</b>") } 
else { document.write(input.classList + "<br>" + "<b>No posee la clase que buscas") };

<input id="Nombre" class="clase1 clase2 clase3" type="text" />
<br>
Clases que posee:

if you want to use the this attribute of an element you give it focus

     $("input[type=email],input[type=password],input[type=text]").focusout(function () {
         if (this.classList.contains("obligatorio")) {  
             console.log("el input tiene clase obligatorio")
         } else {
             console.log("el input tiene clase obligatorio")
             }

        });

it worked for me to validate fields with class