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How to validate mobile number using Javscript

<!DOCTYPE html>
<html>
<head>
<title>Validate mobile number Javascript</title>
</head>
<body>
<input id="txtPhoneNo" type="text" onkeypress="return isNumber(event)" />
<input type="button" value="Submit" onclick="ValidateNo();">
<script>
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;
}
</script>
</body>
</html>

Run it yourself