javascript cell number validation

If number is your form element, then its length will be undefined since elements don't have length. You want

if (number.value.length != 10) { ... }

An easier way to do all the validation at once, though, would be with a regex:

var val = number.value
if (/^\d{10}$/.test(val)) {
    // value is ok, use it
} else {
    alert("Invalid number; must be ten digits")
    number.focus()
    return false
}

\d means "digit," and {10} means "ten times." The ^ and $ anchor it to the start and end, so something like asdf1234567890asdf does not match.

function IsMobileNumber(txtMobId) {
    var mob = /^[1-9]{1}[0-9]{9}$/;
    var txtMobile = document.getElementById(txtMobId);
    if (mob.test(txtMobile.value) == false) {
        alert("Please enter valid mobile number.");
        txtMobile.focus();
        return false;
    }
    return true;
}

Calling Validation Mobile Number Function HTML Code -

Mobile number Validation using Java Script, This link will provide demo and more information.

function isNumber(evt) {
  evt = (evt) ? evt : window.event;
  var charCode = (evt.which) ? evt.which : evt.keyCode;
  if (charCode > 31 && (charCode < 48 || charCode > 57)) {
    alert("Please enter only Numbers.");
    return false;
  }

  return true;
}

function ValidateNo() {
  var phoneNo = document.getElementById('txtPhoneNo');

  if (phoneNo.value == "" || phoneNo.value == null) {
    alert("Please enter your Mobile No.");
    return false;
  }
  if (phoneNo.value.length < 10 || phoneNo.value.length > 10) {
    alert("Mobile No. is not valid, Please Enter 10 Digit Mobile No.");
    return false;
  }

  alert("Success ");
  return true;
}

<input id="txtPhoneNo" type="text" onkeypress="return isNumber(event)" />
<input type="button" value="Submit" onclick="ValidateNo();">