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Show error message if non numeric value entered

How to show error message to user if non-numeric values are entered in the text box.
The error message can be displayed below the text box something like enter the numbers only
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)) {
return false;
}
return true;
}
<input type="text" class="textfield" onkeypress="return isNumber(event)" />

Just add some sort of output in your if. Like alert or even better, with a message near the input field. But this is up to you.
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)) {
alert("only input numbers");
return false;
}
return true;
}
<input type="text" class="textfield" onkeypress="return isNumber(event)" />

Create a dom element to show the error and add a class to initially hide it. If any input is anything other than number remove the class from the element
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)) {
document.getElementById("errorNumber").classList.remove('error-hide')
return false;
}
document.getElementById("errorNumber").classList.add('error-hide')
return true;
}
.error {
color: red;
}
.error-hide {
display: none;
}
<input type="text" class="textfield" onkeyup="isNumber(event)" />
<p id="errorNumber" class='error error-hide'>Please enter only number </p>

You can use a span element to display the messages inline in the page:
function isNumber(evt) {
var errorClass = document.querySelector('.error');
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)) {
errorClass.style.display = 'block';
return false;
}
errorClass.style.display = 'none';
return true;
}
.error{
display: none;
color: red;
}
<input type="text" class="textfield" onkeypress="return isNumber(event)" />
<span class='error' >Enter numbers only</span>

Seems like your are not considering digits when it's typed from keyboard's 'numpad'.
Following should work.
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || (charCode > 57 && charCode < 96) || charCode > 105)) {
document.getElementById("errorNumber").classList.remove('error-hide')
return false;
}
document.getElementById("errorNumber").classList.add('error-hide')
return true;
}
.error {
color: red;
}
.error-hide {
display: none;
}
<input type="text" class="textfield" onkeyup="isNumber(event)" />
<p id="errorNumber" class='error error-hide'>Please enter only number </p>

function isNumber(evt) {
if(isNaN(evt.key)) {
document.getElementById('inptErr').style.display = "block";
} else {
document.getElementById('inptErr').style.display = "none";
}
}
<input type="text" class="textfield" onkeypress="isNumber(event)" />
<span id="inptErr" style="display:none;color:red;">Please enter numbers only</span>

auto multiplication text javascript and auto reduction

<input type="text" id="txt1" onkeyup="sum();" onkeypress="return hanyaAngka(event);"/>
<script>
function hanyaAngka(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return false;
return true;
}
</script>
how to automatically mulitiply everytime i press keyboard
so if i input 1 the result is 1000
input 2 the result is 2000 etc

Here is a basic solution.
var sumkeys = 0;
function sum() {
console.log("The sum is: " + sumkeys);
sumkeys = 0;
}
function hanyaAngka(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode < 48 || charCode > 57)
return 0;
else {
return sumkeys += (charCode - 48) * 1000;
}
}
<input type="text" id="txt1" onkeyup="sum();" onkeypress="return hanyaAngka(event);" />

How my JS function would restrict specific data ?

In asp.net textbox I have called a javascript function which I wrote to restrict only 'digits' entry in text box but I also want to allow '+' sign but can't solve it.
This is what I have tried so far.
function isNumberKey(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return false;
return true;
}

Just reverse your condition and add the ASCII code for + which is 43 to return true
function isNumberKey(evt) {
var charCode = (evt.which) ? evt.which : event.keyCode
if ((charCode >=48 && charCode <= 57) || (charCode == 43))
return true;
return false;
}

If you were able to restrict digits, you can also enter '+', I guess you need ASCII table and corresponding value i.e. how you were restricting digits, and charCode > 31 seems useless if you are checking for digits only:
http://www.asciitable.com/
so if you want to check for +
charCode != 43

You need to check for + as well explicitly, modify your if to :
if (charCode > 31 && (charCode < 48 || charCode > 57) && charCode != 43)
Here is working example:
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57) && charCode != 43) {
return false;
}
return true;
}
<input type="text" class="textfield" value="" id="extra7" name="extra7" onkeypress="return isNumber(event)" />

Working Demo
Ascii Table
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)&& charCode != 43) {
return false;
}
return true;
}

Javascript validation works in Google Chrome but not in Firefox

Here is the js code
function isCharKey(evt){
var charCode = event.keyCode
if ((charCode > 64 && charCode <91 )|| (charCode >96 && charCode<123) || (charCode==32))
return true;
return false;
}
Here is the html code
<label id="exe_form_name">Name:</label><input type="text" name="tbcust_name" id="name1" onkeypress="return isCharKey(event);">

Change
var charCode = event.keyCode
to
var charCode = evt.keyCode

function isCharKey(evt)
{
var charCode = evt.keyCode
if ((charCode > 64 && charCode <91 )|| (charCode >96 && charCode<123) || (charCode==32))
return true;
return false;
}

Javascript Function to enter only alphabets on keypress

I want to enter only character values inside a <textarea> and numeric values in another. I have been able to make a JavaScript function which only allows numeric values to be entered in the <textarea> using onkeypress. This works in Firefox and Chrome.
For alphabets I am creating another JavaScript function using windows.event property. Only problem is this works only in Chrome and not in Firefox.
I want to know how to allow only alphabets to be entered using onkeypress event as used for entering only numeric values?
function isNumberKey(evt){ <!--Function to accept only numeric values-->
//var e = evt || window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode
if (charCode != 46 && charCode > 31
&& (charCode < 48 || charCode > 57))
return false;
return true;
}
function ValidateAlpha(evt)
{
var keyCode = (evt.which) ? evt.which : evt.keyCode
if ((keyCode < 65 || keyCode > 90) && (keyCode < 97 || keyCode > 123) && keyCode != 32)
return false;
return true;
}
<label for="cname" class="label">The Risk Cluster Name</label>
<textarea id="cname" rows="1px" cols="20px" style="resize:none" placeholder="Cluster Name" onKeyPress="return ValidateAlpha(event);"></textarea>
<br>
<label for="cnum">Risk Cluster Number:</label>
<textarea id="cmun" rows="1px" cols="12px" style="resize:none" placeholder="Cluster Number" onkeypress="return isNumberKey(event)"></textarea>

function lettersOnly()
{
var charCode = event.keyCode;
if ((charCode > 64 && charCode < 91) || (charCode > 96 && charCode < 123) || charCode == 8)
return true;
else
return false;
}
<input type="text" name="fname" value="" onkeypress="return lettersOnly(event)"/>

If you don't need to support older browsers I would use the input event. This way you can also catch non-alpha characters if the user pastes text into the textarea.
I cleaned up your HTML a little bit. The most important changes are to the events on cname and cnum. Note that the event in both cases has been changed to oninput.
<label for="cname" class="label"> The Risk Cluster Name</label>
<textarea id="cname" rows="1" cols="20" style="resize:none" placeholder="Cluster Name" oninput="validateAlpha();"></textarea>
<label for="cnum">Risk Cluster Number:</label>
<textarea id="cmun" rows="1" cols="12" style="resize:none" placeholder="Cluster Number" oninput="isNumberKey();"></textarea><br /><br /><br />
Assuming you want cname to only accept characters in the alphabet and cnum to only accept numbers, your JavaScript should be:
function validateAlpha(){
var textInput = document.getElementById("cname").value;
textInput = textInput.replace(/[^A-Za-z]/g, "");
document.getElementById("cname").value = textInput;
}
function isNumberKey(){
var textInput = document.getElementById("cmun").value;
textInput = textInput.replace(/[^0-9]/g, "");
document.getElementById("cmun").value = textInput;
}
This code uses regular expressions, a way to match patterns in strings.

Best Uses
<input type="text" name="checkno" id="checkno" class="form-control" value="" onkeypress="return isNumber(event)"/>
<input type="text" name="checkname" id="checkname" class="form-control" value="" onkeypress="return isAlfa(event)"/>
function isNumber(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57)) {
return false;
}
return true;
}
function isAlfa(evt) {
evt = (evt) ? evt : window.event;
var charCode = (evt.which) ? evt.which : evt.keyCode;
if (charCode > 31 && (charCode < 65 || charCode > 90) && (charCode < 97 || charCode > 122)) {
return false;
}
return true;
}

function digitonly(input,event){
var keyCode = event.which ? event.which : event.keyCode;
var lisShiftkeypressed = event.shiftKey;
if(lisShiftkeypressed && parseInt(keyCode) != 9){return false;}
if((parseInt(keyCode)>=48 && parseInt(keyCode)<=57) || keyCode==37/*LFT ARROW*/ || keyCode==39/*RGT ARROW*/ || keyCode==8/*BCKSPC*/ || keyCode==46/*DEL*/ || keyCode==9/*TAB*/ || keyCode==45/*minus sign*/ || keyCode==43/*plus sign*/){return true;}
BootstrapDialog.alert("Enter Digits Only");
input.focus();
return false;
}
function alphaonly(input,event){
var keyCode = event.which ? event.which : event.keyCode;
//Small Alphabets
if(parseInt(keyCode)>=97 && parseInt(keyCode)<=122){return true;}
//Caps Alphabets
if(parseInt(keyCode)>=65 && parseInt(keyCode)<=90){return true;}
if(parseInt(keyCode)==32 || parseInt(keyCode)==13 || parseInt(keyCode)==46 || keyCode==9/*TAB*/ || keyCode==8/*BCKSPC*/ || keyCode==37/*LFT ARROW*/ || keyCode==39/*RGT ARROW*/ ){return true;}
BootstrapDialog.alert("Only Alphabets are allowed")
input.focus();
return false;
}

hi try below code it worked for me in all browsers, it allows numbers and few special characters like,.+-() :
in the textbox use as follows
<asp:Textbox Id="txtPhone" runat="server" onKeyPress="return onlyNumbersandSpecialChar()"> </asp:Textbox>
function onlyNumbersandSpecialChar(evt) {
var e = window.event || evt;
var charCode = e.which || e.keyCode;
if (charCode > 31 && (charCode < 48 || charCode > 57 || charCode > 107 || charCode > 219 || charCode > 221) && charCode != 40 && charCode != 32 && charCode != 41 && (charCode < 43 || charCode > 46)) {
if (window.event) //IE
window.event.returnValue = false;
else //Firefox
e.preventDefault();
}
return true;
}
</script>