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What is wrong with my simple math equation

I am trying to create a simple calculator that will return the amount of sales tax plus the amount after tax. If I input $80 and the sales tax is 7.5% I should get back an amount of $86 but instead I am getting $806 returned to me. I am unsure why.
//calculation
var total = (itemCost + (itemCost * salesTax/100));
total = Math.round(total)

you need var total = (parseInt(itemCost) + (parseInt(itemCost) * salesTax/100));

The thing is that when you retrieve the value, it is a string. Parse it as a number value by doing:
var salesTax = Number(document.getElementById("salesTax").value);

When you are getting the values of the text fields you have strings. Javascript does a lot of type inferring, that's why it almost works. Your expression gets translated to:
(String)"80" + ((String)"80"*(int)0.075)
(String)"80" + (int)6
And now the String operator+(String, Any) is called which is defined as string concatenation. Therefore you end up with "80" + "6" => "806".
If you for example would write:
"80" * "1" + "80" * "7.5"/"100" you would indeed get the expected result of 86.
But to be sure everything works as expected you should indeed parse the values from a string value to a number value (with parseInt resp. parseFloat).

Format a number to replace a decimal with space

I have a field on a form in Adobe that automatically sums the values from six other fields. The sum results in a decimal "." being placed so there are always 2 digits following.
The values in each of the six fields are typed in using a space to act as decimal place, as in "14 50" = $14.50 (see image).
The resulting sum is correct, but I'd like to add custom formatting that will replace the "." with a space, so that the sum in my example would read "87 00" rather than "87.00"
The javascript must be code that will work in the Adobe Pro custom format option for the field.

var num = 87.27;
var formatted;
if (num % 1 === 0) {
formatted = num.toString() +' 00';
} else {
formatted = num.toString().replace('.', ' ');
}

Append extra zeroes to decimal to make 4.5 look 4.500 but 4.5234 should be 4.5234

I have a value fetched from the database, it's like:
4.5 which should be 4.500
0.01 which should be 0.010
11 which should be 11.000
so I used this piece of code
sprintf("%.3f",(double)$html['camp_cpc'])
But here arised another problem. If $html['camp_cpc'] = '4.5234', then also it displays 4.523 instead of original value 4.5234
Also for other values with larger decimal like 0.346513, its only showing up to 0.346.
How can I solve this problem in JavaScript also?

Floats 4.5 and 4.500 correspond to the same number, so they cannot (and should not) be used/stored in a way that preserves the different representation. If you need to preserve the original representation given by a user, you need to store this field as a list (string) and convert to a float whenever you need the float value

In Javascript at least, this is an implementation of what I think you want:
function getValue(x, points) {
var str = x.toString();
// Convert to string
var idx = str.indexOf(".");
// If the number is an integer
if(!~idx) return str + "." + "0".repeat(points);
// Get the tail of the number
var end = str.substr(idx+1);
// If the tail exceeds the number of decimal places, return the full string
if(end.length > points) return str;
// Otherwise return the int + the tail + required number of zeroes
return str.substr(0, idx) + "." + end.substr(0, points) + "0".repeat(points-end.length);
}
console.log(getValue(4.5, 3)); //4.500
console.log(getValue(0.01, 3)); //0.010
console.log(getValue(11, 3)); //11.000
Working demo (Makes use of ES6 String.repeat for demonstration purposes)
The important thing to note here is that this is string manipulation. Once you start to say "I want the number to look like..." it's no longer a number, it's what you want to show the user.
This takes your number, converts it to the string and pads the end of the string with the appropriate number of zeroes. If the decimal exceeds the number of places required the full number is returned.

In PHP, use %0.3f — and you don't need to cast as (double)
<?php
echo sprintf("%0.3f", 4.5); // "4.500"
echo sprintf("%0.3f", 4.5234); // "4.523"
If you want to display 4 decimal places, use %0.4f
echo sprintf("%0.4f", 4.5); // "4.5000"
echo sprintf("%0.4f", 4.5234); // "4.5234"
To do this in JavaScript
(4.5).toFixed(3); // "4.500"

It could look sth. like this:
var n = [4.5234, 0.5, 0.11, 456.45];
var temp_n;
for(var i = 0; i < n.length; i++) {
temp_n = String(n[i]).split(".");
if(temp_n[1] == null || temp_n[1].length < 3) {
n[i] = n[i].toFixed(3);
}
}

trim trailing zeros not quite working

I have a number of prices on a page that arein 4 decimal places. Some products are priced in 4 decimal places, but some are only in 2.
At the moment our website is set to display in 4 decimal places for every product and I have to use javascript to trim down the prices on those products that aren't 2 decimal places.
So I have prices like so...
£0.1234
£1.1000
£10.9900
£100.0000
I have the following javascript which works fine for prices that have a number greater than 1 after the decimal point, but it fails on prices where there are just 0's after the decimal point...
$.each($("#mydiv"),function(){
var price = $(this).text().replace("£","");
var number = parseFloat(price);
var integerPart = number.toString().split(".")[0] == 0 ? 0: number.toString().split(".")[0].length;
var decimalPart = number.toString().split(".")[1].length;
if(decimalPart > 2){
$(this).text("£" + number.toPrecision(integerPart + decimalPart));
}else{
$(this).text("£" + number.toPrecision(integerPart + 2));
}
});
The ones it fails on are prices like £100.0000 - I would like the prices to appear as follows - no rounding...
£0.1234
£1.10
£10.99
£100.00

Just use a regexp to remove any trailing zeroes if the preceeding characters are the decimal period followed by another two digits:
$('.myClass').text(function(_, t) {
return t.replace(/(\.\d\d)00$/, '$1');
});
NB: you can't use duplicate element ID's, so your $.each call should be moot. If there really are multiple fields that this needs doing to, mark them with a class, not an ID. The .text call in the code above will automatically cope with multiple elements.
EDIT if you really can't upgrade your jQuery:
$('.myClass').each(function() {
var $this = $(this);
var text = $this.text();
text = text.replace(/(\.\d\d)00$/, '$1');
$this.text(text);
});

Additional to Alnitak's answer, I would highly recommend to store the prices of your store's items as integers (or maybe in your case longs), so you don't have to worry about imprecisions of double or float.
Because you have 4 decimal places the integers (or longs) can not represent pence, but 1/100 pence.
Example:
Price on website | price in database
-------------------------------------------
£0.1234 | 1234
£1.10 | 11000
£10.99 | 109900
£100.00 | 1000000

How to store more than 10 digit number in javascript using the var?

First of all,
What am i doing ?
I have to set the limit of emails in our product in webpage.It's handled with the javascript for validation.It handles upto 8 digit numbers fine. But in our QA team enters the more than 17 digit number in the text box of other email field.It throw the negative message.What can i do ???
My sample code is:
if(form.otherEmails) {
if(validEmailArray.endsWith(',')){
var otherEmailLength = validEmailArray.substring(0,validEmailArray.length-1).split(",");
var setLimitOtherEmail = window.parent.document.getElementById('setLimitOtherEmail').value;
if(setLimitOtherEmail == '-1'){
form.otherEmails.value = otherEmailLength;
}
else if(otherEmailLength.length <= setLimitOtherEmail){
form.otherEmails.value = otherEmailLength;
}
else{
alert("More than "+setLimitOtherEmail+ " " +"Recipient emailIds not allowed in this section.\nIf you want to send it to more recipients, Please create a Bulk Contact Group.");
form.otherEmails.focus();
return false;
}
}
else
form.otherEmails.value = validEmailArray;
}

This is due to the limit being a string, and when a string is being compared to a number (length) the number is coerced into a string, not the other way around.
These are then compared lexicographically - and lexicographically "9" is more (>) than "19".
You need to use parseInt(setLimitOtherEmail, 10) to get the value as a number before comparing them.

Try parsing each of the numbers into Integers before performing any comparison operations on them.
var setLimitOtherEmail = parseInt(window.parent.document.getElementById('setLimitOtherEmail').value);
Other than that are you certain otherEmailLength is actually the number that you want? From the looks of it you are taking the substring of validEmail array and splitting it on "," but it doesn't look like you actually get the length of the array. Try adding .length to the end of the value of otherEmailLength.
var otherEmailLength = validEmailArray.substring(0,validEmailArray.length-1).split(",").length;