Return False not working inside jQuery.ajax

Question

P.S.: Read \"EDITED on 2019-06-29\":

I have a webform for updating user information, and when he updates his email a verification is performed via

Answer 1

<form action="yourpage" method="post" onsubmit="return matchpass();">
  <div>
   <label> Name</label>
   <input type="text" name="name" id="name">
  </div>
  <div>
   <label> Email ID</label>
   <input type="email" name="email" id="email">            
  </div>
  <div>
   <label> Mobile No</label>
   <input type="text" name="mob"  maxlength="10" onkeyup="check_if_exists();" autocomplete="off" id="mob">
  </div>                             
  <div>
   <button type="button" >Send</button>
  </div>
  <span id="err"></span>
  <div>
   <label> OTP</label>
   <input type="password" name="otp" id="otp"  maxlength="6" placeholder="****">
   <span id="err2"></span>
  </div>    
  <div>
   <input type="reset" value="Reset" class="reset-btn">
   <input type="submit" name="submit" id="submit" value="Submit" >              
  </div>
</form>
<input type="hidden"  id="otpcheck"/>



<script>
function matchpass()
{

     $.ajax({   
    type:"post",
    url: "yourpage",
    data:{ mobile:mob,otp:otp},
    success:function(data)
    {
       if(data==1)
       {
           document.getElementById("otpcheck").value=1; //important
       }else{

           document.getElementById("err2").style.color = "red";
           document.getElementById("err2").innerHTML = "invalid OTP Number ";
         document.getElementById("otpcheck").value=0; //important
       }


    }
    });

    if(document.getElementById("otpcheck").value==0){
        return false;
    }

}