sample.js

1) ActiveModel validation in rails server side :

class MyObject < ActiveRecord::Base
  validates :state, :presence => true
  validates :other_state, :presence => true, if: state.persent?
end

2) Lets say we have drop down with following state : 

<select id="stateDropdown" name="state">
  <option value="state1">State1</option>
  <option value="state2">State2</option>
  <option value="state3">State3</option>
  <option value="otherState">OtherState</option>  
</select>
<input id="otherStateInput" type="text" name="other_state" readonly>

3) Javascript code to validate other state when user has selected `other` in state dropdown

// otherStatePresence function will check other state is selected, apply validator to input and exceutes validation
function otherStatePresence(e){
  var form,form_validators;
  if( $("#stateDropdown option:selected").text().toLowerCase() == "otherstate"){
      form = $("#otherStateInput").closest("form")
      form_validators = ClientSideValidations.forms[form.attr("id")].validators
      form_validators["other_state"] = {"presence": [{"message": "can't be blank"}]}
    }else{
      form = $("#otherStateInput").closest("form")
      form_validators = ClientSideValidations.forms[form.attr("id")].validators
      delete form_validators["other_state"]      
    }
    form.resetClientSideValidations();
    return form.isValid(form_validators);
}

//Bind event on state dropdown, and don't allow user when user has selected state other than "OtherState"
$("#stateDropdown").on("change",function(event){
  if( $("#stateDropdown option:selected").text().toLowerCase() == "otherstate"){
    $("#otherStateInput").attr("readonly",false)
  }else{
    $("#otherStateInput").attr("value","")
    $("#otherStateInput").attr("readonly",true)
    $("#otherStateInput").attr("data-validate",false)
  }
});

//check other state stuff before submitting form to server
$("#myObjectForm").submit(otherStatePresence);