We Keep Coding

Convert a string to a big integer in Javascript?

I am trying to convert a string to a big integer to perform some arithmetic calculations. However, when I try this:
Number("9007199254740993")
...I am getting this unexpected result:
9007199254740992
I suspect that this is probably because of the limit on the size of integers that Number is capable of working with.
Basically, I want to check if two strings are consecutive numbers or not. Since Number is not returning the correct value, I am getting the incorrect difference for "9007199254740993" and "9007199254740992". Specifically, I am expecting 1, but getting 0.
One possibility I considered is dividing each number by a factor to make each of them smaller. Is there any other solution?

Javascript's Number type is a numeric data type in the double-precision 64-bit floating point format (IEEE 754).
If you are dealing with large integers, use a BigInt or a corresponding library.

I you don't want to rely on BigInt and only have positive integers in mind, you can also write the successor test yourself. Full code in the snippet below.
Notes
A string representation of a positive integer is easily convertible to a decimal array where the index represents the exponent to the base 10. For example "42" ~> [2, 4] (since 42 = 2*10^0 + 4*10^1). You can also just as easily convert it back.
Now for the successor test you just need to define the increment operation (which is just adding 1 with carry). With that you can just compare if the increment of one number is equal to the unincremented other number (and vice versa).
Code
// Convert a string representation of positive decimal integer to an array of decimals.
const toArray = numberString => Array.from(numberString, c => parseInt(c))
.reverse();
// Convert the array representation of a positive decimal integer string back to the corresponding string representation (this is the inverse of `toArray`).
const fromArray = numberArray => numberArray.map(String)
.reverse()
.join('');
console.log(fromArray(toArray("9007199254740993")) === "9007199254740993"); // true
// Perform the increment operation on the array representation of the positive decimal integer.
const increment = numberArray => {
let carry = 1;
const incrementedNumberArray = [];
numberArray.forEach(i => {
let j;
if (carry === 0) {
j = i;
} else if (carry === 1) {
if (i === 9) {
j = 0;
} else {
j = i + 1;
carry = 0;
}
}
incrementedNumberArray.push(j);
});
if (carry === 1) {
incrementedNumberArray.push(1);
}
return incrementedNumberArray;
};
console.log(fromArray(increment(toArray("9007199254740993"))) === "9007199254740994"); // true
console.log(fromArray(increment(toArray("9999999999999999"))) === "10000000000000000"); // true
// Test if two strings represent positive integers where one is the other's successor.
const isSuccessor = (a, b) => {
const a_ = increment(toArray(a));
const b_ = increment(toArray(b));
return fromArray(a_) === b || fromArray(b_) === a;
};
console.log(isSuccessor("9007199254740993", "9007199254740994")); // true
console.log(isSuccessor("9007199254740994", "9007199254740993")); // true
console.log(isSuccessor("9999999999999999", "10000000000000000")); // true
console.log(isSuccessor("10000000000000000", "9999999999999999")); // true
console.log(isSuccessor("10000000000000000", "10000000000000002")); // false

You can use BIG integer library like one in JAVA.
check here
npm install big-integer
var bigInt = require("big-integer");
var largeNumber1 = bigInt("9007199254740993");
var largeNumber2 = bigInt("9007199254740994"); // any other number
var ans = largeNumber1.minus(largeNumber2);
if(ans == 1 || ans == -1){
console.log('consecutive ')
}else{
console.log('not consecutive ')
}

Note: I recommend you to use BigInt(as suggested by #Andreas in comment), if you are dealing with Big Numbers.
UPDATED
Use this code to compare big positive integers(The arguments should be in string format)
function compareBigNumber(num1, num2) {
if (num1 > Number.MAX_SAFE_INTEGER && num2 > Number.MAX_SAFE_INTEGER) {
var newNum1 = num1.split('').reverse();
var newNum2 = num2.split('').reverse();
do {
newNum1.pop();
newNum2.pop();
} while (newNum1[newNum1.length-1] === '0' || newNum2[newNum2.length-1] === '0')
return compareBigNumber(newNum1.reverse().join(''), newNum2.reverse().join(''));
} else if(num1 > Number.MAX_SAFE_INTEGER){
return 'num1 is greater'
} else if (num2 > Number.MAX_SAFE_INTEGER) {
return 'num2 is greater'
}
else {
var num1Int = parseInt(num1);
var num2Int = parseInt(num2);
if (num1Int > num2Int) {
return 'Num1 is greater';
} else if (num2Int > num1Int){
return 'Num2 is greater'
} else {
return 'Num1 is equal to Num2';
}
}
}
console.log(compareBigNumber("9007199254740992", "9007199254740993"))
console.log(compareBigNumber("100000000000000000000", "0"))

convert formatted currency amount to a double value using regex

I have different currency format that I want to convert into a double value. Example:
1,000,000.00 => 1000000.00
2'345',00 => 2345.00
2'344'334.03 => 1000000.03
I have the following solution which works and is very inefficient.I am trying to figure out some regex way of doing it.
decimalPlace = amount[amount.length - 3];
if (decimalPlace === '.' && amount.indexOf(',') < -1 && amount.indexOf("'") < -1) {
return amount
}
if (decimalPlace === ',' && amount.indexOf("'") < -1) {
value = amount.split('.').join('')
.replace(',', '.')
return value
}
if (decimalPlace === '.' && amount.indexOf(',') > -1) {
value = amount.split(',').join('')
return value
}
if (decimalPlace === ',' && amount.indexOf("'") > -1) {
value = amount.split("'").join('')
.replace(',', '.')
return value
}
if (decimalPlace === '.' && amount.indexOf("'") > -1) {
value = amount.split("'").join('')
return value
}
return amount
I would appreciate any suggestion with this issue.

You've probably made this more complicated then it has to be.
If those are the only formats that need to be supported, you could use a function like this:
var v1 = "1,000,000.00";
var v2 = "2'345',00";
var v3 = "2'344'334.03";
var v4 = "2,00"
function format(str){
if(str.replace(/[^,]/g, "").length === 1){ // Contains only one comma.
str = str.replace(",", ".");
}
if(str.indexOf("'")!=-1){
str = str.replace(/'/g, "").replace(",", ".");
}
return parseFloat(str.replace(/,/g, "")).toFixed(2);
}
[v1, v2, v3, v4].forEach(v => console.log(format(v)));
EDIT: I've added support for values with only one comma.
EDIT 2: Actually, it seems that I've made that mistake myself (making thing complicated).
I think all this can be as simple as:
var v1 = "1,000,000.00";
var v2 = "2'345',00";
var v3 = "2'344'334.03";
var v4 = "2,00"
function format(str){
if(str.replace(/[^,]/g, "").length === 1){ // Contains only one comma.
str = str.replace(",", ".");
}
return parseFloat(str.replace(/[^0-9\.]/g, "")).toFixed(2);
}
[v1, v2, v3, v4].forEach(v => console.log(format(v)));

I'd recommend against relying too much on regular expressions here. The way I'd approach this is in 2 parts.
First, work out what the decimal point is. This will change based on locale but it's very easy to work it out - just create a function which will add a localised decimal point and then return it.
function getDecimalPoint(locale) {
return (1.2).toLocaleString(locale).replace(/\d+/g, "");
}
The benefit of this function is that if you pass in no locale, the current one will be used (which is "en-GB" for me).
Now that you can identify a decimal point, you can use that to split the formatted number into 2 parts - evenything before the decimal (whole numbers) and everything after it (decimals).
function unformatNumber(number, locale) {
// Split the number based on the decimal point.
var numberParts = String(number).split(getDecimalPoint(locale));
// Remove everything that isn't a number from the whole numbers and parse
// it as a number.
var unformatted = Number(numberParts[0].replace(/\D+/g, ""));
// Check to see if there are any decimals. If there are, convert them into
// a decimal and add them to the unformatted result.
var decimals = numberParts[1];
if (decimals && decimals.length) {
unformatted += decimals / Math.pow(10, decimals.length);
}
return unformatted;
}
To use it, simply pass your numbers to the function.
unformatNumber("1,000,000.00"); // -> 1000000
unformatNumber("2'345',00", "fr"); // -> 2345
unformatNumber("2'344'334.03"); // -> 2344344.03
// I know those numbers aren't formatted in French, but you can change your
// format as necessary.
function getDecimalPoint(locale) {
return (1.2).toLocaleString(locale).replace(/\d+/g, "");
}
function unformatNumber(number, locale) {
var numberParts = String(number).split(getDecimalPoint(locale));
var unformatted = Number(numberParts[0].replace(/\D+/g, ""));
var decimals = numberParts[1];
if (decimals && decimals.length) {
unformatted += decimals / Math.pow(10, decimals.length);
}
return unformatted;
}
console.log(unformatNumber("1,000,000.00"));
console.log(unformatNumber("2'345',00", "fr"));
console.log(unformatNumber("2'344'334.03"));

Move comma position JavaScript

I'm trying to move the position of the comma with the use of JavaScript. I have managed to remove all the parts of the string I needed removing. The only problem is that the comma is in the wrong position.
The current outcome is 425.00, but I simply want '42.50'
success: function(result) {
if (result != '') {
alert(" "+result+" ");
}
var discountVal = result.replace(/\D/g,'');
newDiscountVal = discountVal.replace(7.50, '');
$("input#amount").val(discountVal);
}
I am grabbing database echo values with a combination of string and echo - numbers..

You could divide by ten, then convert back to a String using toFixed(2) which forces formatting of 2 decimal places
Javascript allows implicit conversion of Strings to numbers, by firstly converting the String to a Number so it is valid to divide a String by a number.
var input= "4250.00";
var output = (original / 100).toFixed(2); // => "42.50"
Note this method has different behaviour due to rounding. Consider the case 9.99. If you use a string manipulation technique you'll get ".99", with divide by 10 method above you'll get "1.00". However from what has been said in comments I believe your inputs always end .00 and never anything else, so there will be no difference in reality.

If it is number you can just divide by 10
If it is string you can do like this:
var ind = text.indexOf('.');
text = text.replace('.', '');
text.slice(0, ind-1) + '.' + text.slice(ind-1, text.length)

Here is a solution:
function moveComma(val, moveCommaByInput) {
if (val || typeof val === 'number') {
const valueNumber = Number(val);
const moveCommaBy = moveCommaByInput || 0;
if (isNaN(valueNumber)) {
return null;
} else {
return Number(`${valueNumber}e${moveCommaBy}`);
}
}
return null;
}

This is how i solved it..
var discountVal = result.replace(/\D/g, '');
var newDiscountVal = discountVal.replace(7.50, '');
var lastDigits = newDiscountVal.substr(newDiscountVal.length - 2);
var removedDigits = newDiscountVal.slice(0,newDiscountVal.length - 2);
var discountRealValue = removedDigits + '.' + lastDigits;
$("input#amount").val(discountRealValue);
Cheers

Culture sensitive ParseFloat Function in JavaScript?

Do anyone have suggestion for writing culture sensitive ParseFloat Function in JavaScript, So that when I have a string 100,000.22 in US culture format the parse float function returns 100000.22 whereas if I enter 100.000,22 in Swedish Culture it returns 100000.22 in float?

I've improved mwilcox' function to handle values withous separators.
function parseFloatOpts (str) {
if(typeof str === "number"){
return str;
}
var ar = str.split(/\.|,/);
var value = '';
for (var i in ar) {
if (i>0 && i==ar.length-1) {
value += ".";
}
value +=ar[i];
}
return Number(value);
}

This is a bit rough-and-ready, but it may be sufficient, allowing you to pass in the thousands and decimal separators:
function parseFloatOpts(num, decimal, thousands) {
var bits = num.split(decimal, 2),
ones = bits[0].replace(new RegExp('\\' + thousands, 'g'), '');
ones = parseFloat(ones, 10),
decimal = parseFloat('0.' + bits[1], 10);
return ones + decimal;
}
Examples:
parseFloatOpts("100.000,22", ',', '.'); //100000.22
parseFloatOpts("100,000.22", '.', ','); //100000.22
NB that this doesn't ensure that the thousands separator really does represent thousands, etc., or do lots of other safeguarding that you may wish to do, depending on the importance of the function.

var parse = function(st){
if(st.indexOf(",") === st.length-3){
st = st.replace(".", "").replace(",", ".");
}else{
st = st.replace(",", "");
}
return parseFloat(st, 10)
}
console.log(parse("100,000.22")) // 100000.22
console.log(parse("100.000,22")) // 100000.22
I'm just checking if there is a comma in the 3rd-to-last position. This could be further refined to check if there is a period in the 4th to last position in the case thee is no comma (such as 100.000)

Looking at lonesomday's gave me this thought:
You could also do:
function parse (str)
var ar = str.split(/\.|,/);
return Number(ar[0]+ar[1]+"."+ar[3]);

Here is a rough function. It will assume the last punctuation to indicate decimals, whether it is a comma, period, or any other character you may need to indicate. It then eliminates other punctuations from the whole number. Puts it back together and parses as float.
function normalizeFloat(number, chars) {
var lastIndex = -1;
for(i=0; i < chars.length; i++) {
t = number.lastIndexOf(chars[i]);
if (t > lastIndex) {
lastIndex = t;
}
}
if (lastIndex == -1) {
lastIndex = number.length;
}
var whole = number.substring(0, lastIndex);
var precision = number.substring(lastIndex);
for (i=0; i < chars.length; i++) {
whole = whole.replace(chars[i], '');
precision = precision.replace(chars[i],'.');
}
number = whole + precision;
f = parseFloat(number);
return f;
}
try this:
alert(normalizeFloat('12.345,77', [',','.']).toFixed(2));
alert(normalizeFloat('12,345.77', [',','.']).toFixed(2));

Need your current Group and Decimal Separator from Culture Info.
function escapeRegExp(string) {
return string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
}
function parseFloatOpts(str, groupSeparator, decimalSeparator) {
if (typeof str === "number") {
return str;
}
var value = str.replace(new RegExp(escapeRegExp(groupSeparator), 'g'), "");
value = value.replace(decimalSeparator, ".");
return Number(value);
}

If you really for displaying and/or parsing floats (or dates or currencies or more) in different locales for JavaScript, then my recommendation is the GlobalizeJS (https://github.com/globalizejs/globalize) library.
It's a bit tough to set up at first (at least it was in my experience), but totally recommended for proper management of this matter.

How to convert decimal to hexadecimal in JavaScript

How do you convert decimal values to their hexadecimal equivalent in JavaScript?

Convert a number to a hexadecimal string with:
hexString = yourNumber.toString(16);
And reverse the process with:
yourNumber = parseInt(hexString, 16);

If you need to handle things like bit fields or 32-bit colors, then you need to deal with signed numbers. The JavaScript function toString(16) will return a negative hexadecimal number which is usually not what you want. This function does some crazy addition to make it a positive number.
function decimalToHexString(number)
{
if (number < 0)
{
number = 0xFFFFFFFF + number + 1;
}
return number.toString(16).toUpperCase();
}
console.log(decimalToHexString(27));
console.log(decimalToHexString(48.6));

The code below will convert the decimal value d to hexadecimal. It also allows you to add padding to the hexadecimal result. So 0 will become 00 by default.
function decimalToHex(d, padding) {
var hex = Number(d).toString(16);
padding = typeof (padding) === "undefined" || padding === null ? padding = 2 : padding;
while (hex.length < padding) {
hex = "0" + hex;
}
return hex;
}

function toHex(d) {
return ("0"+(Number(d).toString(16))).slice(-2).toUpperCase()
}

For completeness, if you want the two's-complement hexadecimal representation of a negative number, you can use the zero-fill-right shift >>> operator. For instance:
> (-1).toString(16)
"-1"
> ((-2)>>>0).toString(16)
"fffffffe"
There is however one limitation: JavaScript bitwise operators treat their operands as a sequence of 32 bits, that is, you get the 32-bits two's complement.

With padding:
function dec2hex(i) {
return (i+0x10000).toString(16).substr(-4).toUpperCase();
}

The accepted answer did not take into account single digit returned hexadecimal codes. This is easily adjusted by:
function numHex(s)
{
var a = s.toString(16);
if ((a.length % 2) > 0) {
a = "0" + a;
}
return a;
}
and
function strHex(s)
{
var a = "";
for (var i=0; i<s.length; i++) {
a = a + numHex(s.charCodeAt(i));
}
return a;
}
I believe the above answers have been posted numerous times by others in one form or another. I wrap these in a toHex() function like so:
function toHex(s)
{
var re = new RegExp(/^\s*(\+|-)?((\d+(\.\d+)?)|(\.\d+))\s*$/);
if (re.test(s)) {
return '#' + strHex( s.toString());
}
else {
return 'A' + strHex(s);
}
}
Note that the numeric regular expression came from 10+ Useful JavaScript Regular Expression Functions to improve your web applications efficiency.
Update: After testing this thing several times I found an error (double quotes in the RegExp), so I fixed that. HOWEVER! After quite a bit of testing and having read the post by almaz - I realized I could not get negative numbers to work.
Further - I did some reading up on this and since all JavaScript numbers are stored as 64 bit words no matter what - I tried modifying the numHex code to get the 64 bit word. But it turns out you can not do that. If you put "3.14159265" AS A NUMBER into a variable - all you will be able to get is the "3", because the fractional portion is only accessible by multiplying the number by ten(IE:10.0) repeatedly. Or to put that another way - the hexadecimal value of 0xF causes the floating point value to be translated into an integer before it is ANDed which removes everything behind the period. Rather than taking the value as a whole (i.e.: 3.14159265) and ANDing the floating point value against the 0xF value.
So the best thing to do, in this case, is to convert the 3.14159265 into a string and then just convert the string. Because of the above, it also makes it easy to convert negative numbers because the minus sign just becomes 0x26 on the front of the value.
So what I did was on determining that the variable contains a number - just convert it to a string and convert the string. This means to everyone that on the server side you will need to unhex the incoming string and then to determine the incoming information is numeric. You can do that easily by just adding a "#" to the front of numbers and "A" to the front of a character string coming back. See the toHex() function.
Have fun!
After another year and a lot of thinking, I decided that the "toHex" function (and I also have a "fromHex" function) really needed to be revamped. The whole question was "How can I do this more efficiently?" I decided that a to/from hexadecimal function should not care if something is a fractional part but at the same time it should ensure that fractional parts are included in the string.
So then the question became, "How do you know you are working with a hexadecimal string?". The answer is simple. Use the standard pre-string information that is already recognized around the world.
In other words - use "0x". So now my toHex function looks to see if that is already there and if it is - it just returns the string that was sent to it. Otherwise, it converts the string, number, whatever. Here is the revised toHex function:
/////////////////////////////////////////////////////////////////////////////
// toHex(). Convert an ASCII string to hexadecimal.
/////////////////////////////////////////////////////////////////////////////
toHex(s)
{
if (s.substr(0,2).toLowerCase() == "0x") {
return s;
}
var l = "0123456789ABCDEF";
var o = "";
if (typeof s != "string") {
s = s.toString();
}
for (var i=0; i<s.length; i++) {
var c = s.charCodeAt(i);
o = o + l.substr((c>>4),1) + l.substr((c & 0x0f),1);
}
return "0x" + o;
}
This is a very fast function that takes into account single digits, floating point numbers, and even checks to see if the person is sending a hex value over to be hexed again. It only uses four function calls and only two of those are in the loop. To un-hex the values you use:
/////////////////////////////////////////////////////////////////////////////
// fromHex(). Convert a hex string to ASCII text.
/////////////////////////////////////////////////////////////////////////////
fromHex(s)
{
var start = 0;
var o = "";
if (s.substr(0,2).toLowerCase() == "0x") {
start = 2;
}
if (typeof s != "string") {
s = s.toString();
}
for (var i=start; i<s.length; i+=2) {
var c = s.substr(i, 2);
o = o + String.fromCharCode(parseInt(c, 16));
}
return o;
}
Like the toHex() function, the fromHex() function first looks for the "0x" and then it translates the incoming information into a string if it isn't already a string. I don't know how it wouldn't be a string - but just in case - I check. The function then goes through, grabbing two characters and translating those in to ASCII characters. If you want it to translate Unicode, you will need to change the loop to going by four(4) characters at a time. But then you also need to ensure that the string is NOT divisible by four. If it is - then it is a standard hexadecimal string. (Remember the string has "0x" on the front of it.)
A simple test script to show that -3.14159265, when converted to a string, is still -3.14159265.
<?php
echo <<<EOD
<html>
<head><title>Test</title>
<script>
var a = -3.14159265;
alert( "A = " + a );
var b = a.toString();
alert( "B = " + b );
</script>
</head>
<body>
</body>
</html>
EOD;
?>
Because of how JavaScript works in respect to the toString() function, all of those problems can be eliminated which before were causing problems. Now all strings and numbers can be converted easily. Further, such things as objects will cause an error to be generated by JavaScript itself. I believe this is about as good as it gets. The only improvement left is for W3C to just include a toHex() and fromHex() function in JavaScript.

Without the loop:
function decimalToHex(d) {
var hex = Number(d).toString(16);
hex = "000000".substr(0, 6 - hex.length) + hex;
return hex;
}
// Or "#000000".substr(0, 7 - hex.length) + hex;
// Or whatever
// *Thanks to MSDN
Also isn't it better not to use loop tests that have to be evaluated?
For example, instead of:
for (var i = 0; i < hex.length; i++){}
have
for (var i = 0, var j = hex.length; i < j; i++){}

Combining some of these good ideas for an RGB-value-to-hexadecimal function (add the # elsewhere for HTML/CSS):
function rgb2hex(r,g,b) {
if (g !== undefined)
return Number(0x1000000 + r*0x10000 + g*0x100 + b).toString(16).substring(1);
else
return Number(0x1000000 + r[0]*0x10000 + r[1]*0x100 + r[2]).toString(16).substring(1);
}

Constrained/padded to a set number of characters:
function decimalToHex(decimal, chars) {
return (decimal + Math.pow(16, chars)).toString(16).slice(-chars).toUpperCase();
}

For anyone interested, here's a JSFiddle comparing most of the answers given to this question.
And here's the method I ended up going with:
function decToHex(dec) {
return (dec + Math.pow(16, 6)).toString(16).substr(-6)
}
Also, bear in mind that if you're looking to convert from decimal to hex for use in CSS as a color data type, you might instead prefer to extract the RGB values from the decimal and use rgb().
For example (JSFiddle):
let c = 4210330 // your color in decimal format
let rgb = [(c & 0xff0000) >> 16, (c & 0x00ff00) >> 8, (c & 0x0000ff)]
// Vanilla JS:
document.getElementById('some-element').style.color = 'rgb(' + rgb + ')'
// jQuery:
$('#some-element').css('color', 'rgb(' + rgb + ')')
This sets #some-element's CSS color property to rgb(64, 62, 154).

var number = 3200;
var hexString = number.toString(16);
The 16 is the radix and there are 16 values in a hexadecimal number :-)

function dec2hex(i)
{
var result = "0000";
if (i >= 0 && i <= 15) { result = "000" + i.toString(16); }
else if (i >= 16 && i <= 255) { result = "00" + i.toString(16); }
else if (i >= 256 && i <= 4095) { result = "0" + i.toString(16); }
else if (i >= 4096 && i <= 65535) { result = i.toString(16); }
return result
}

If you want to convert a number to a hexadecimal representation of an RGBA color value, I've found this to be the most useful combination of several tips from here:
function toHexString(n) {
if(n < 0) {
n = 0xFFFFFFFF + n + 1;
}
return "0x" + ("00000000" + n.toString(16).toUpperCase()).substr(-8);
}

AFAIK comment 57807 is wrong and should be something like:
var hex = Number(d).toString(16);
instead of
var hex = parseInt(d, 16);
function decimalToHex(d, padding) {
var hex = Number(d).toString(16);
padding = typeof (padding) === "undefined" || padding === null ? padding = 2 : padding;
while (hex.length < padding) {
hex = "0" + hex;
}
return hex;
}

And if the number is negative?
Here is my version.
function hexdec (hex_string) {
hex_string=((hex_string.charAt(1)!='X' && hex_string.charAt(1)!='x')?hex_string='0X'+hex_string : hex_string);
hex_string=(hex_string.charAt(2)<8 ? hex_string =hex_string-0x00000000 : hex_string=hex_string-0xFFFFFFFF-1);
return parseInt(hex_string, 10);
}

As the accepted answer states, the easiest way to convert from decimal to hexadecimal is var hex = dec.toString(16). However, you may prefer to add a string conversion, as it ensures that string representations like "12".toString(16) work correctly.
// Avoids a hard-to-track-down bug by returning `c` instead of `12`
(+"12").toString(16);
To reverse the process you may also use the solution below, as it is even shorter.
var dec = +("0x" + hex);
It seems to be slower in Google Chrome and Firefox, but is significantly faster in Opera. See http://jsperf.com/hex-to-dec.

I'm doing conversion to hex string in a pretty large loop, so I tried several techniques in order to find the fastest one. My requirements were to have a fixed-length string as a result, and encode negative values properly (-1 => ff..f).
Simple .toString(16) didn't work for me since I needed negative values to be properly encoded. The following code is the quickest I've tested so far on 1-2 byte values (note that symbols defines the number of output symbols you want to get, that is for 4-byte integer it should be equal to 8):
var hex = ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'];
function getHexRepresentation(num, symbols) {
var result = '';
while (symbols--) {
result = hex[num & 0xF] + result;
num >>= 4;
}
return result;
}
It performs faster than .toString(16) on 1-2 byte numbers and slower on larger numbers (when symbols >= 6), but still should outperform methods that encode negative values properly.

Converting hex color numbers to hex color strings:
A simple solution with toString and ES6 padStart for converting hex color numbers to hex color strings.
const string = `#${color.toString(16).padStart(6, '0')}`;
For example:
0x000000 will become #000000
0xFFFFFF will become #FFFFFF
Check this example in a fiddle here

How to convert decimal to hexadecimal in JavaScript
I wasn't able to find a brutally clean/simple decimal to hexadecimal conversion that didn't involve a mess of functions and arrays ... so I had to make this for myself.
function DecToHex(decimal) { // Data (decimal)
length = -1; // Base string length
string = ''; // Source 'string'
characters = [ '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' ]; // character array
do { // Grab each nibble in reverse order because JavaScript has no unsigned left shift
string += characters[decimal & 0xF]; // Mask byte, get that character
++length; // Increment to length of string
} while (decimal >>>= 4); // For next character shift right 4 bits, or break on 0
decimal += 'x'; // Convert that 0 into a hex prefix string -> '0x'
do
decimal += string[length];
while (length--); // Flip string forwards, with the prefixed '0x'
return (decimal); // return (hexadecimal);
}
/* Original: */
D = 3678; // Data (decimal)
C = 0xF; // Check
A = D; // Accumulate
B = -1; // Base string length
S = ''; // Source 'string'
H = '0x'; // Destination 'string'
do {
++B;
A& = C;
switch(A) {
case 0xA: A='A'
break;
case 0xB: A='B'
break;
case 0xC: A='C'
break;
case 0xD: A='D'
break;
case 0xE: A='E'
break;
case 0xF: A='F'
break;
A = (A);
}
S += A;
D >>>= 0x04;
A = D;
} while(D)
do
H += S[B];
while (B--)
S = B = A = C = D; // Zero out variables
alert(H); // H: holds hexadecimal equivalent

You can do something like this in ECMAScript 6:
const toHex = num => (num).toString(16).toUpperCase();

If you are looking for converting Large integers i.e. Numbers greater than Number.MAX_SAFE_INTEGER -- 9007199254740991, then you can use the following code
const hugeNumber = "9007199254740991873839" // Make sure its in String
const hexOfHugeNumber = BigInt(hugeNumber).toString(16);
console.log(hexOfHugeNumber)

To sum it all up;
function toHex(i, pad) {
if (typeof(pad) === 'undefined' || pad === null) {
pad = 2;
}
var strToParse = i.toString(16);
while (strToParse.length < pad) {
strToParse = "0" + strToParse;
}
var finalVal = parseInt(strToParse, 16);
if ( finalVal < 0 ) {
finalVal = 0xFFFFFFFF + finalVal + 1;
}
return finalVal;
}
However, if you don't need to convert it back to an integer at the end (i.e. for colors), then just making sure the values aren't negative should suffice.

I haven't found a clear answer, without checks if it is negative or positive, that uses two's complement (negative numbers included). For that, I show my solution to one byte:
((0xFF + number +1) & 0x0FF).toString(16);
You can use this instruction to any number bytes, only you add FF in respective places. For example, to two bytes:
((0xFFFF + number +1) & 0x0FFFF).toString(16);
If you want cast an array integer to string hexadecimal:
s = "";
for(var i = 0; i < arrayNumber.length; ++i) {
s += ((0xFF + arrayNumber[i] +1) & 0x0FF).toString(16);
}

In case you're looking to convert to a 'full' JavaScript or CSS representation, you can use something like:
numToHex = function(num) {
var r=((0xff0000&num)>>16).toString(16),
g=((0x00ff00&num)>>8).toString(16),
b=(0x0000ff&num).toString(16);
if (r.length==1) { r = '0'+r; }
if (g.length==1) { g = '0'+g; }
if (b.length==1) { b = '0'+b; }
return '0x'+r+g+b; // ('#' instead of'0x' for CSS)
};
var dec = 5974678;
console.log( numToHex(dec) ); // 0x5b2a96

This is based on Prestaul and Tod's solutions. However, this is a generalisation that accounts for varying size of a variable (e.g. Parsing signed value from a microcontroller serial log).
function decimalToPaddedHexString(number, bitsize)
{
let byteCount = Math.ceil(bitsize/8);
let maxBinValue = Math.pow(2, bitsize)-1;
/* In node.js this function fails for bitsize above 32bits */
if (bitsize > 32)
throw "number above maximum value";
/* Conversion to unsigned form based on */
if (number < 0)
number = maxBinValue + number + 1;
return "0x"+(number >>> 0).toString(16).toUpperCase().padStart(byteCount*2, '0');
}
Test script:
for (let n = 0 ; n < 64 ; n++ ) {
let s=decimalToPaddedHexString(-1, n);
console.log(`decimalToPaddedHexString(-1,${(n+"").padStart(2)}) = ${s.padStart(10)} = ${("0b"+parseInt(s).toString(2)).padStart(34)}`);
}
Test results:
decimalToPaddedHexString(-1, 0) = 0x0 = 0b0
decimalToPaddedHexString(-1, 1) = 0x01 = 0b1
decimalToPaddedHexString(-1, 2) = 0x03 = 0b11
decimalToPaddedHexString(-1, 3) = 0x07 = 0b111
decimalToPaddedHexString(-1, 4) = 0x0F = 0b1111
decimalToPaddedHexString(-1, 5) = 0x1F = 0b11111
decimalToPaddedHexString(-1, 6) = 0x3F = 0b111111
decimalToPaddedHexString(-1, 7) = 0x7F = 0b1111111
decimalToPaddedHexString(-1, 8) = 0xFF = 0b11111111
decimalToPaddedHexString(-1, 9) = 0x01FF = 0b111111111
decimalToPaddedHexString(-1,10) = 0x03FF = 0b1111111111
decimalToPaddedHexString(-1,11) = 0x07FF = 0b11111111111
decimalToPaddedHexString(-1,12) = 0x0FFF = 0b111111111111
decimalToPaddedHexString(-1,13) = 0x1FFF = 0b1111111111111
decimalToPaddedHexString(-1,14) = 0x3FFF = 0b11111111111111
decimalToPaddedHexString(-1,15) = 0x7FFF = 0b111111111111111
decimalToPaddedHexString(-1,16) = 0xFFFF = 0b1111111111111111
decimalToPaddedHexString(-1,17) = 0x01FFFF = 0b11111111111111111
decimalToPaddedHexString(-1,18) = 0x03FFFF = 0b111111111111111111
decimalToPaddedHexString(-1,19) = 0x07FFFF = 0b1111111111111111111
decimalToPaddedHexString(-1,20) = 0x0FFFFF = 0b11111111111111111111
decimalToPaddedHexString(-1,21) = 0x1FFFFF = 0b111111111111111111111
decimalToPaddedHexString(-1,22) = 0x3FFFFF = 0b1111111111111111111111
decimalToPaddedHexString(-1,23) = 0x7FFFFF = 0b11111111111111111111111
decimalToPaddedHexString(-1,24) = 0xFFFFFF = 0b111111111111111111111111
decimalToPaddedHexString(-1,25) = 0x01FFFFFF = 0b1111111111111111111111111
decimalToPaddedHexString(-1,26) = 0x03FFFFFF = 0b11111111111111111111111111
decimalToPaddedHexString(-1,27) = 0x07FFFFFF = 0b111111111111111111111111111
decimalToPaddedHexString(-1,28) = 0x0FFFFFFF = 0b1111111111111111111111111111
decimalToPaddedHexString(-1,29) = 0x1FFFFFFF = 0b11111111111111111111111111111
decimalToPaddedHexString(-1,30) = 0x3FFFFFFF = 0b111111111111111111111111111111
decimalToPaddedHexString(-1,31) = 0x7FFFFFFF = 0b1111111111111111111111111111111
decimalToPaddedHexString(-1,32) = 0xFFFFFFFF = 0b11111111111111111111111111111111
Thrown: 'number above maximum value'
Note: Not too sure why it fails above 32 bitsize

rgb(255, 255, 255) // returns FFFFFF
rgb(255, 255, 300) // returns FFFFFF
rgb(0,0,0) // returns 000000
rgb(148, 0, 211) // returns 9400D3
function rgb(...values){
return values.reduce((acc, cur) => {
let val = cur >= 255 ? 'ff' : cur <= 0 ? '00' : Number(cur).toString(16);
return acc + (val.length === 1 ? '0'+val : val);
}, '').toUpperCase();
}

Arbitrary precision
This solution take on input decimal string, and return hex string. A decimal fractions are supported. Algorithm
split number to sign (s), integer part (i) and fractional part (f) e.g for -123.75 we have s=true, i=123, f=75
integer part to hex:
if i='0' stop
get modulo: m=i%16 (in arbitrary precision)
convert m to hex digit and put to result string
for next step calc integer part i=i/16 (in arbitrary precision)
fractional part
count fractional digits n
multiply k=f*16 (in arbitrary precision)
split k to right part with n digits and put them to f, and left part with rest of digits and put them to d
convert d to hex and add to result.
finish when number of result fractional digits is enough
// #param decStr - string with non-negative integer
// #param divisor - positive integer
function dec2HexArbitrary(decStr, fracDigits=0) {
// Helper: divide arbitrary precision number by js number
// #param decStr - string with non-negative integer
// #param divisor - positive integer
function arbDivision(decStr, divisor)
{
// algorithm https://www.geeksforgeeks.org/divide-large-number-represented-string/
let ans='';
let idx = 0;
let temp = +decStr[idx];
while (temp < divisor) temp = temp * 10 + +decStr[++idx];
while (decStr.length > idx) {
ans += (temp / divisor)|0 ;
temp = (temp % divisor) * 10 + +decStr[++idx];
}
if (ans.length == 0) return "0";
return ans;
}
// Helper: calc module of arbitrary precision number
// #param decStr - string with non-negative integer
// #param mod - positive integer
function arbMod(decStr, mod) {
// algorithm https://www.geeksforgeeks.org/how-to-compute-mod-of-a-big-number/
let res = 0;
for (let i = 0; i < decStr.length; i++)
res = (res * 10 + +decStr[i]) % mod;
return res;
}
// Helper: multiply arbitrary precision integer by js number
// #param decStr - string with non-negative integer
// #param mult - positive integer
function arbMultiply(decStr, mult) {
let r='';
let m=0;
for (let i = decStr.length-1; i >=0 ; i--) {
let n = m+mult*(+decStr[i]);
r= (i ? n%10 : n) + r
m= n/10|0;
}
return r;
}
// dec2hex algorithm starts here
let h= '0123456789abcdef'; // hex 'alphabet'
let m= decStr.match(/-?(.*?)\.(.*)?/) || decStr.match(/-?(.*)/); // separate sign,integer,ractional
let i= m[1].replace(/^0+/,'').replace(/^$/,'0'); // integer part (without sign and leading zeros)
let f= (m[2]||'0').replace(/0+$/,'').replace(/^$/,'0'); // fractional part (without last zeros)
let s= decStr[0]=='-'; // sign
let r=''; // result
if(i=='0') r='0';
while(i!='0') { // integer part
r=h[arbMod(i,16)]+r;
i=arbDivision(i,16);
}
if(fracDigits) r+=".";
let n = f.length;
for(let j=0; j<fracDigits; j++) { // frac part
let k= arbMultiply(f,16);
f = k.slice(-n);
let d= k.slice(0,k.length-n);
r+= d.length ? h[+d] : '0';
}
return (s?'-':'')+r;
}
// -----------
// TESTS
// -----------
let tests = [
["0",2],
["000",2],
["123",0],
["-123",0],
["00.000",2],
["255.75",5],
["-255.75",5],
["127.999",32],
];
console.log('Input Standard Abitrary');
tests.forEach(t=> {
let nonArb = (+t[0]).toString(16).padEnd(17,' ');
let arb = dec2HexArbitrary(t[0],t[1]);
console.log(t[0].padEnd(10,' '), nonArb, arb);
});
// Long Example (40 digits after dot)
let example = "123456789012345678901234567890.09876543210987654321"
console.log(`\nLong Example:`);
console.log('dec:',example);
console.log('hex: ',dec2HexArbitrary(example,40));

The problem basically how many padding zeros to expect.
If you expect string 01 and 11 from Number 1 and 17. it's better to use Buffer as a bridge, with which number is turn into bytes, and then the hex is just an output format of it. And the bytes organization is well controlled by Buffer functions, like writeUInt32BE, writeInt16LE, etc.
import { Buffer } from 'buffer';
function toHex(n) { // 4byte
const buff = Buffer.alloc(4);
buff.writeInt32BE(n);
return buff.toString('hex');
}
> toHex(1)
'00000001'
> toHex(17)
'00000011'
> toHex(-1)
'ffffffff'
> toHex(-1212)
'fffffb44'
> toHex(1212)
'000004bc'

Here's my solution:
hex = function(number) {
return '0x' + Math.abs(number).toString(16);
}
The question says: "How to convert decimal to hexadecimal in JavaScript". While, the question does not specify that the hexadecimal string should begin with a 0x prefix, anybody who writes code should know that 0x is added to hexadecimal codes to distinguish hexadecimal codes from programmatic identifiers and other numbers (1234 could be hexadecimal, decimal, or even octal).
Therefore, to correctly answer this question, for the purpose of script-writing, you must add the 0x prefix.
The Math.abs(N) function converts negatives to positives, and as a bonus, it doesn't look like somebody ran it through a wood-chipper.
The answer I wanted, would have had a field-width specifier, so we could for example show 8/16/32/64-bit values the way you would see them listed in a hexadecimal editing application. That, is the actual, correct answer.