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HTML and Javscript field returning NaN, when field is blank

I am working on a webapp, and the formatting of the one text field is working almost perfectly, however, the only thing that is happening I do not want to happen is when I click out of the field and there is no value, it returns NaN
HTML
<input type="text" class="rounded" name="sname" id="investamt" onblur="handleChange()"><br>
Javascript
var fnf = document.getElementById("investamt");
fnf.addEventListener('keyup', function(evt){
var n = parseInt(this.value.replace(/\D/g,''),10);
fnf.value = n.toLocaleString();
}, false);
function handleChange() {
var myValue = document.getElementById("investamt").value;
if (myValue.indexOf("$") != 0)
{
myValue = "$" + myValue;
}
document.getElementById("investamt").value = myValue;
}
Why is it returning NaN when I click out of the text field and it is blank, I would like it to return 0 instead, for example.

You can check the value and zero it out with if (isNaN(n)) n = 0;
var fnf = document.getElementById("investamt");
fnf.addEventListener('keyup', function(evt) {
var n = parseInt(this.value.replace(/\D/g, ''), 10);
if (isNaN(n)) n = 0;
fnf.value = n.toLocaleString();
}, false);
function handleChange() {
var myValue = document.getElementById("investamt").value;
if (myValue.indexOf("$") != 0) {
myValue = "$" + myValue;
}
document.getElementById("investamt").value = myValue;
}
<input type="text" class="rounded" name="sname" id="investamt" onblur="handleChange()"><br>

when you parse something empty string it will return NAN. try this on console:--
parseInt(''.replace(/\D/g,''),10)

try this code....
hope it will fill your requirments :)
var fnf = document.getElementById("investamt");
fnf.addEventListener('keyup', function(evt){
var n
n = this.value.replace(/\D/g,'')
fnf.value = n;
}, false);
function handleChange() {
var myValue = document.getElementById("investamt").value;
if(myValue.length === 0){ myValue = '0'}
if (myValue.indexOf("$") != 0)
{
myValue = "$" + myValue;
}
document.getElementById("investamt").value = myValue;
}
<input type="text" class="rounded" name="sname" id="investamt" onblur="handleChange()"><br>

how to replace input numbers with commas after key presses

I want to replace a number over 100 with commas. Like 1000 to 1,000 and 1000000 to 1,000,000 etc. in HTML. I have found the code on here to do so but it only works with predetermined numbers being passed. I don't want it to work for a predetermined number but for any number typed into the box.
<label for="turnover">Estimated Monthly Card Turnover:</label><br />
<span>£ </span><input type="text" id="turnover" maxlength="11"
name="turnover" size="10" required>*
<br /><br />
<script type="text/javascript">
$('#turnover').keydown(function(){
var str = $(this).val();
str = str.replace(/\D+/g, '');
$(this).val(str.replace(/\B(?=(\d{3})+(?!\d))/g, ","));});
</script>

I created a solution using pure javascript.
function onChange(el) {
var newValue = el.value.replace(/,/g, '');
var count = 0;
const last = newValue.substring(newValue.length - 1, newValue.length); // last input value
// check if last input value is real a number
if (!isNumber(last)) {
el.value = el.value.substring(0, el.value.length - 1);
return;
}
newValue = newValue.split('')
.reverse().map((it) => {
var n = it;
if (count > 0 && count % 3 == 0) n = n + ',';
count++;
return n;
})
.reverse().join('')
el.value = newValue
// document.getElementById('value').innerHTML = newValue
}
function isNumber(input) {
return input.match(/\D/g) == undefined;
}
<label>Number</label>
<input id="numbers" onkeyup="onChange(this)">

There are a couple of issues with your code:
It runs once when the page loads, not after that. I added a button to fix that.
The id used in your code does not match the actual id of the input field.
Input fields must be read and written using .val(). .text() works only for divs, spans etc.
Note that the conversion now works one time, after that it fails to properly parse the new text which now contains the comma(s).
function numberWithCommas(x) {
return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",");
}
function ShowComma() {
console.clear();
var val = parseInt($("#comma").val());
console.log(val);
val = numberWithCommas(val);
console.log(val);
$("#comma").val(val);
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<label for="turnover">Estimated Monthly Card Turnover:</label><br />
<span>£ </span><input type="value" id="comma" maxlength="30" name="turnover" size="10" required>*
<button onclick="ShowComma()">Show Comma</button>

To finalise this I have putgetElementById functions in so that this will work with a wordpress contact form 7. This must be with a text field though as it will not work with the number field as it will now accept commas:
<script>
document.getElementById("averagetrans").onkeyup = function() {onChange(this)};
document.getElementById("Turnover").onkeyup = function() {onChange(this)};
</script>
<script type="text/javascript">
function onChange(el) {
var newValue = el.value.replace(/,/g, '');
var count = 0;
const last = newValue.substring(newValue.length - 1, newValue.length); // last input value
// check if last input value is real a number
if (!isNumber(last)) {
el.value = el.value.substring(0, el.value.length - 1);
return;
}
newValue = newValue.split('')
.reverse().map((it) => {
var n = it;
if (count > 0 && count % 3 == 0) n = n + ','; // put commas into numbers 1000 and over
count++;
return n;
})
.reverse().join('')
el.value = newValue
// document.getElementById('value').innerHTML = newValue
}
function isNumber(input) {
return input.match(/\D/g) == undefined;
}
</script>

Formatting number as thousand using only Javascript

Console.log is showing the correct result, but how can I add the same formatting to the input type while typing.
Input type is reset after every comma to zero.
1000 to 1,000
Please Help.
This code is working here
function numberWithCommas(number) {
if (isNaN(number)) {
return '';
}
var asString = '' + Math.abs(number),
numberOfUpToThreeCharSubstrings = Math.ceil(asString.length / 3),
startingLength = asString.length % 3,
substrings = [],
isNegative = (number < 0),
formattedNumber,
i;
if (startingLength > 0) {
substrings.push(asString.substring(0, startingLength));
}
for (i=startingLength; i < asString.length; i += 3) {
substrings.push(asString.substr(i, 3));
}
formattedNumber = substrings.join(',');
if (isNegative) {
formattedNumber = '-' + formattedNumber;
}
document.getElementById('test').value = formattedNumber;
}
<input type="number" id="test" class="test" onkeypress="numberWithCommas(this.value)">

Some notes:
Because you want commas, the type is not a number, it's a string
Because you want to work on the input after you type, it's onkeyup not onkeypressed
I have a solution that does a regex replace for 3 characters with 3 characters PLUS a comma:
var x = "1234567";
x.replace(/.../g, function(e) { return e + ","; } );
// Gives: 123,456,7
i.e. almost the right answer, but the commas aren't in the right spot. So let's fix it up with a String.prototype.reverse() function:
String.prototype.reverse = function() {
return this.split("").reverse().join("");
}
function reformatText() {
var x = document.getElementById('test').value;
x = x.replace(/,/g, ""); // Strip out all commas
x = x.reverse();
x = x.replace(/.../g, function(e) { return e + ","; } ); // Insert new commas
x = x.reverse();
x = x.replace(/^,/, ""); // Remove leading comma
document.getElementById('test').value = x;
}
<input id="test" class="test" onkeyup="reformatText()">

function numberWithCommas(x) {
var real_num = x.toString().replace(/(\d)(?=(\d{3})+(?!\d))/g, "$1,");
console.log(real_num);
document.getElementById('test').value = real_num;
}
<input type="number" id="test" onkeypress="numberWithCommas(this.value)">

Check out my fiddle here http://jsfiddle.net/6cqn3uLf/
You'd need another regex to limit to numbers but this will format based on the user's locale - which may be advantageous here.
<input id="mytext" type="text">
$(function () {
$('#btnformat').on('input propertychange paste', function () {
var x = $('#btnformat').val();
$('#btnformat').val(Number(x.replace(/,/g,'')).toLocaleString());
});
});

if jquery is not overhead for your application then you can use
https://code.google.com/p/jquery-numberformatter/

how to get dynamic id of dynamically created textbox in jquery

i want to perform keyup event via textbox id, and all textbox are dynamically created with onclick button event. for this i have to make 20 keyup function. if i use 20 keyup function then my code will become too lengthy and complex. instead of this i want to use a common function for all textbox. can anybody suggest me how to do it..thanks
here is what i am doing to solve it:
<div class="input_fields_wrap">
<button class="add_field_button">Add Booking</button></div>
<div id='TextBoxesGroup'>
<div id="TextBoxDiv1">
</div>
</div>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<script>
$(document).ready(function() {
var counter = 2;
$(".add_field_button").click(function() {
if (counter > 10) {
alert("Only 10 textboxes allow");
return false;
}
var newTextBoxDiv = $(document.createElement('div'))
.attr("id", 'TextBoxDiv' + counter);
newTextBoxDiv.after().html('<div id="target"><label>Textbox #' + counter + ' : </label>' +
'<input type="text" name="textbox' + counter +
'" id="firsttextbox' + counter + '" value="" > <input type="text" name="textbox' + counter +
'" id="secondtextbox' + counter + '" value="" > Remove<input type="text" id="box' + counter + '" value="">sum</div>');
newTextBoxDiv.appendTo("#TextBoxesGroup");
counter++;
});
function check(a, b) {
var first = a;
var second = b;
var temp = temp;
var novalue = "";
result = parseInt(first) + parseInt(second);
if (!isNaN(result)) {
return result;
} else {
return novalue;
}
}
$(this).on("keyup", "#firsttextbox2", function(e) {
e.preventDefault();
var a = document.getElementById('firsttextbox2').value;
var b = document.getElementById('secondtextbox2').value;
var number = 2;
result = check(a, b);
document.getElementById('box2').value = result;
});
$(this).on("keyup", "#firsttextbox3", function(e) {
var number = 3;
e.preventDefault();
var a = document.getElementById('firsttextbox3').value;
var b = document.getElementById('secondtextbox3').value;
result = check(a, b);
document.getElementById('box3').value = result;
});
$(this).on("keyup", "#firsttextbox4", function(e) {
var number = 4;
e.preventDefault();
var a = document.getElementById('firsttextbox4').value;
var b = document.getElementById('secondtextbox4').value;
result = check(a, b);
final = document.getElementById('box4').value = result;
});
$(this).on("keyup", "#secondtextbox2", function(e) {
e.preventDefault();
var a = document.getElementById('firsttextbox2').value;
var b = document.getElementById('secondtextbox2').value;
result = check(a, b);
document.getElementById('box2').value = result;
});
$(this).on("keyup", "#secondtextbox3", function(e) {
e.preventDefault();
var a = document.getElementById('firsttextbox3').value;
var b = document.getElementById('secondtextbox3').value;
result = check(a, b);
document.getElementById('box3').value = result;
});
$(this).on("keyup", "#secondtextbox4", function(e) {
e.preventDefault();
var a = document.getElementById('firsttextbox4').value;
var b = document.getElementById('secondtextbox4').value;
result = check(a, b);
document.getElementById('box4').value = result;
});
$(this).on("click", "#remove_field", function(e) { //user click on remove text
e.preventDefault();
$(this).parent('#target').remove();
counter--;
});
});
</script>

See the snippet below to see how you can make this implementation more modular and useable. The trick is to think: what do I want to do? I want to be able to add multiple inputs and add their value, printing the result in another input.
It comes down to using classes - since we are going to use the same kind of thing for every row. Then apply something that works for all classes. No IDs whatsoever! You can even use the name property of the input that contains the value you want to save. Using the [] in that property will even pass you back a nice array when POSTING!
I know this looks like a daunting lot, but remove my comments and the number of lines reduces dramatically and this kind of code is almost infinitely extendable and reusable.
But have a look, this works and its simple and - most of all - it's DRY (don't repeat yourself 0 once you do, re-evaluate as there should be a better way!)!
Update
You could also use a <ol>as a wrapper and then add an <li> to this every time, so you get automatic counting of boxes in the front end without any effort from your end! Actually, thats so nice for this that I have changed my implementation.
var add = $('#add_boxes');
var all = $('#boxes');
var amountOfInputs = 2;
var maximumBoxes = 10;
add.click(function(event){
// create a limit
if($(".box").length >= maximumBoxes){
alert("You cannot have more than 10 boxes!");
return;
}
var listItem = $('<li class="box"></li>');
// we will add 2 boxes here, but we can modify this in the amountOfBoxes value
for(var i = 0; i < amountOfInputs; i++){
listItem.append('<input type="text" class="input" />');
}
listItem.append('<input type="text" class="output" name="value" />');
// Lets add a link to remove this group as well, with a removeGroup class
listItem.append('<input type="button" value="Remove" class="removeGroup" />')
listItem.appendTo(all);
});
// This will tie in ANY input you add to the page. I have added them with the class `input`, but you can use any class you want, as long as you target it correctly.
$(document).on("keyup", "input.input", function(event){
// Get the group
var group = $(this).parent();
// Get the children (all that arent the .output input)
var children = group.children("input:not(.output)");
// Get the input where you want to print the output
var output = group.children(".output");
// Set a value
var value = 0;
// Here we will run through every input and add its value
children.each(function(){
// Add the value of every box. If parseInt fails, add 0.
value += parseInt(this.value) || 0;
});
// Print the output value
output.val(value);
});
// Lets implement your remove field option by removing the groups parent div on click
$(document).on("click", ".removeGroup", function(event){
event.preventDefault();
$(this).parent(".box").remove();
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"></script>
<ol id="boxes">
</ol>
<input type="button" value="Add a row" id="add_boxes" />

You can target all your textboxes, present or future, whatever their number, with a simple function like this :
$(document).on("keyup", "input[type=text]", function(){
var $textbox = $(this);
console.log($textbox.val());
})
$("button").click(function(){
$("#container").append('<input type="text" /><br>');
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="container">
<input type="text" /><br>
<input type="text" /><br>
<input type="text" /><br>
</div>
<button>Create one more</button>
You don't need complicated generated IDs, not necessarily a class (except if you have other input[type=text] you don't want to conflict with). And you don't need to duplicate your code and write 20 times the same function. Ever. If you're duplicating code, you're doing wrong.

Add classes "a" and "b" to the textboxes and "box" to the box. Then add data-idx attribute with the index (unused!?). Finally register the event handlers:
$('.a').on('keyup', function(e){
e.preventDefault();
var $this = $(this)
var $p = $this.parent()
var a= this.value;
var b= $p.find('.b').val()
var number =$this.data('idx') //unused!?
var result = check(a,b)
$p.find('.box').val(result)
})
$('.b').on('keyup', function(e){
e.preventDefault();
var $this = $(this)
var $p = $this.parent()
var a= $p.find('.a').val()
var b= this.value
var result = check(a,b)
$p.find('.box').val(result)
})
Or a general one:
$('.a,.b').on('keyup', function(e){
e.preventDefault();
var $p = $(this).parent()
var a= $p.find('.a').val()
var b= $p.find('.b').val()
var result = check(a,b)
$p.find('.box').val(result)
})

You can assign a class to all textboxes on which you want to perform keyup event and than using this class you can attach the event on elements which have that class. Here is an example
var html="";
for (var i = 0; i < 20; i++)
{
html += "<input type='text' id='txt" + i + "' class='someClass' />";
}
$("#testDiv").html(html);
Attach keyup event on elements which have class someClass.
$(".someClass").keyup(function () {
alert($(this).attr("id"));
});

A little helper to combine with your favorite answer:
var uid = function () {
var id = 0;
return function () {
return ++id;
};
}();
Usage:
uid(); // 1
uid(); // 2
uid(); // 3

Providing a code-snippet which may give you some hint:
$(".add_field_button").click(function ()
{
if (counter > 10)
{
alert("Only 10 textboxes allow");
return false;
}
var txtBoxDiv = $("<div id='TextBoxDiv"+counter+"' style='float:left;width:10%; position:relative; margin-left:5px;' align='center'></div>");
//creating the risk weight
var txtBox1 = $('<input />',
{
'id' : 'fst_textbox_' + counter,
'name' : 'textbox'+counter,
'type' : 'text',
'class' : 'input_field',
'onClick' : 'txtBoxFun(this,'+counter+')'
});
var txtBox2 = $('<input />',
{
'id' : 'sec_textbox_' + counter,
'name' : 'textbox'+counter,
'type' : 'text',
'class' : 'input_field',
'onClick' : 'txtBoxFun(this,'+counter+')'
});
var txtBox3 = $('<input />',
{
'id' : 'sum_textbox_' + counter,
'name' : 'textbox'+counter,
'type' : 'text',
'class' : 'input_field',
});
$(txtBoxDiv).append(txtBox1).append(txtBox2);
$(txtBoxDiv).append(txtBox3);
});
function txtBoxFun(obj, count)
{
var idGet = $(obj).attr('id');
var idArr = new Array();
idArr = idGet.split("_");
if(idArr[0] == "fst")
{
var sumTxt = parseInt(parseInt($(obj).val()) + parseInt($("#sec_textbox_"+count).val()));
}
else if(idArr[0] == "sec")
{
var sumTxt = parseInt(parseInt($(obj).val()) + parseInt($("#fst_textbox_"+count).val()));
}
$("#sum_textbox_"+count).val(sumTxt);
}

Check if Number entered is correct By ID - JavaScript

Would like to know how to check true and false and in return give error message if checked and the number is incorrect..
<input name="student1" type="text" size="1" id="studentgrade1"/>
<input name="student2" type="text" size="1" id="studentgrade2"/>
<input name="student3" type="text" size="1" id="studentgrade3"/>
so here we have 3 inputbox , now i would like to check the result by entering number into those inputbox.
studentgrade1 = 78
studentgrade2 = 49
studentgrade3 = 90
<< Using JavaScript >>
So If User entered wrong number e.g "4" into inputbox of (studentgrade1) display error..
same for otherinputbox and if entered correct number display message and says.. correct.
http://jsfiddle.net/JxfcH/5/

OK your question is kinda unclear but i am assuming u want to show error
if the input to the text-box is not equal to some prerequisite value.
here is the modified checkGrade function
function checkgrade() {
var stud1 = document.getElementById("studentgrade1");
VAR errText = "";
if (stud1.exists() && (parseInt(stud1.value) == 78){return true;}
else{errText += "stud1 error";}
//do similiar processing for stud2 and stud 3.
alert(errText);
}

See demo →
I think this is what you're looking for, though I would recommend delimiting your "answer sheet" variable with commas and then using split(',') to make the array:
// answers
var result ="756789";
// turn result into array
var aResult = [];
for (var i = 0, il = result.length; i < il; i+=2) {
aResult.push(result[i]+result[i+1]);
}
function checkgrade() {
var tInput,
msg = '';
for (var i = 0, il = aResult.length; i < il; i++) {
tInput = document.getElementById('studentgrade'+(i+1));
msg += 'Grade ' + (i+1) + ' ' +
(tInput && tInput.value == aResult[i] ? '' : 'in') +
'correct!<br>';
}
document.getElementById('messageDiv').innerHTML = msg;
}
See demo →

Try this http://jsfiddle.net/JxfcH/11/
function checkgrade() {
var stud1 = document.getElementById("studentgrade1");
var stud2 = document.getElementById("studentgrade2");
var stud3 = document.getElementById("studentgrade3");
if (((parseInt(stud1.value) == 78)) && ((parseInt(stud2.value) == 49)) && ((parseInt(stud3.value) == 90)))
{
alert("correct");
}
else
{
alert("error correct those values");
}
}