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Can't convert string into number

I have a large text from which I read data according to the scheme. Key words are placed in the "smallArtName" array. The scheme looks like this:
(key word) xxx (cordX|cordY)
I can't convert the string I received to a number. It seems to me that the reason is white space, visible in the terminal in the picture. I tried to use the replace method which works for sample text, but not for my value.
I'm a beginner and I could probably do it simpler, but the code I wrote works, and this is the most important thing for now.
for (i = 0; i < smallArtName.length; i++) {
var n = art.artPrintScreen.indexOf(smallArtName[i]);
if (n > -1) {
var tempString = art.artPrintScreen.substring(n, n + 100);
betweenChar = tempString.indexOf('|');
for (k = betweenChar - 10; k <= betweenChar + 10; k++) {
if (tempString[k] == '(') {
xStart = k;
}
if (tempString[k] == ')') {
yEnd = k;
}
}
cordX = tempString.slice(xStart + 1, betweenChar);
cordY = tempString.slice(betweenChar + 1, yEnd);
strTest = " t est".replace(/\s/g, '')
var cordY2 = cordY.replace(/\s/g, '')
console.log(typeof (cordY))
console.log(cordY2)
console.log(cordY2[0])
console.log(cordY2[1])
console.log(cordY2[2])
console.log(cordY2[3])
console.log(cordY2[4])
console.log(cordY2[5])
console.log(strTest)
var cordYtest = parseInt(cordY2, 10);
console.log(cordYtest)
}
}
Terminal:
-181
-
1
8
1
test
NaN
string
-154
-
1
5
4
test
NaN
string
104
1
0
4
undefined
test
NaN
Fragment of input text:
Ukryta twierdza (Mapa podziemi I) 153 ‭(‭−‭72‬‬|‭−‭155‬‬)‬
Ukryta twierdza (Amfora Mgły VI) 135 ‭(‭73‬|‭104‬)‬
Ukryta twierdza (Mapa podziemi IV) 131 ‭(‭154‬|‭−‭72‬‬)‬

Analysing your sample input strings, I found some unicode characters \u202c and \u202d that should be stripped before converting to number. Also, the negative values are prefixed by the character −, which is different than minus -, se we need to replace it. That being said, all parsing could be done with a single regex:
var input = "Ukryta twierdza (Mapa podziemi I) 153 ‭(‭−‭72‬‬|‭−‭155‬‬)‬";
input = input.replace(/\u202d|\u202c/g, "");
input = input.replace(/−/g, "-");
var m = input.match(/.*\((.*)\)\s*(.+?)\s*\((.+)\|(.+)\)/);
console.log(m);
console.log(parseInt(m[3]));
console.log(parseInt(m[4]));
Explaining the regex:
.* - Something that will be ignored
\((.*)\) - Something enclosed in parenthesis
\s*(.+?)\s* - Something possibly surrounded by spaces
\((.+)\|(.+)\) - Two parts split by a | and enclosed by parenthesis

getting a string length that contains unicode character exceeding 0xffff

I’m using this character, double sharp '𝄪' which unicode is 0x1d12a.
If I use it in a string, I can’t get the correct string length:
str = "F𝄪"
str.length // returns 3, even though there are 2 characters!
How do I get the function to return the correct answer, whether or not I’m using special unicode or not ?

String.prototype.codes = function() { return [...this].length };
String.prototype.chars = function() {
let GraphemeSplitter = require('grapheme-splitter');
return (new GraphemeSplitter()).countGraphemes(this);
}
console.log("F𝄪".codes()); // 2
console.log("👩‍❤️‍💋‍👩".codes()); // 8
console.log("❤️".codes()); // 2
console.log("F𝄪".chars()); // 2
console.log("👩‍❤️‍💋‍👩".chars()); // 1
console.log("❤️".chars()); // 1

That's the function I wrote to get string length in codepoint length
function nbUnicodeLength(string){
var stringIndex = 0;
var unicodeIndex = 0;
var length = string.length;
var second;
var first;
while (stringIndex < length) {
first = string.charCodeAt(stringIndex); // returns an integer between 0 and 65535 representing the UTF-16 code unit at the given index.
if (first >= 0xD800 && first <= 0xDBFF && string.length > stringIndex + 1) {
second = string.charCodeAt(stringIndex + 1);
if (second >= 0xDC00 && second <= 0xDFFF) {
stringIndex += 2;
} else {
stringIndex += 1;
}
} else {
stringIndex += 1;
}
unicodeIndex += 1;
}
return unicodeIndex;
}

To sumarize my comments:
That's just the lenght of that string.
Some chars involve other chars as well, even if it looks like a single character. "̉mủt̉ả̉̉̉t̉ẻd̉W̉ỏ̉r̉̉d̉̉".length == 24
From this (great) blog post, they have a function that will return correct length:
function fancyCount(str){
const joiner = "\u{200D}";
const split = str.split(joiner);
let count = 0;
for(const s of split){
//removing the variation selectors
const num = Array.from(s.split(/[\ufe00-\ufe0f]/).join("")).length;
count += num;
}
//assuming the joiners are used appropriately
return count / split.length;
}
console.log(fancyCount("F𝄪") == 2) // true

Javascript (and Java) strings use UTF-16 encoding.
Unicode codepoint U+0046 (F) is encoded in UTF-16 using 1 codeunit: 0x0046
Unicode codepoint U+1D12A (𝄪) is encoded in UTF-16 using 2 codeunits (known as a "surrogate pair"): 0xD834 0xDD2A
That is why you are getting a length of 3, not 2. The length counts the number of encoded codeunits, not the number of Unicode codepoints.

Alternatives to escape(string) in JavaScript

When changing to TypeScript I'm not allowed to use escape(string) anymore because it's deprecated. The reason I still use it is that the alternatives encodeURI and encodeURIComponent give a different results.
var s = "Å"
console.log(escape(s));
console.log(encodeURI(s));
console.log(encodeURIComponent(s));
I don't use this for URLs, but for a CSV export.
What are other alternatives that will give me the same result as escape(string)?

In EcmaScript spec there is algorithm:
Call ToString(string).
Compute the number of characters in Result(1).
Let R be the empty string.
Let k be 0.
If k equals Result(2), return R.
Get the character at position k within Result(1).
If Result(6) is one of the 69 nonblank ASCII characters ABCDEFGHIJKLMNOPQRSTUVWXYZ
abcdefghijklmnopqrstuvwxyz 0123456789 #*_+-./, go to step 14.
Compute the 16-bit unsigned integer that is the Unicode character encoding of Result(6).
If Result(8), is less than 256, go to step 12.
Let S be a string containing six characters “%uwxyz” where wxyz are four hexadecimal digits encoding the
value of Result(8).
Go to step 15.
Let S be a string containing three characters “%xy” where xy are two hexadecimal digits encoding the
value of Result(8).
Go to step 15.
Let S be a string containing the single character Result(6).
Let R be a new string value computed by concatenating the previous value of R and S.
Increase k by 1.
Go to step 5.
which can be coded like this:
(function(global) {
var allowed = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789#*_+-./,';
global.escapeString = function(str) {
str = str.toString();
var len = str.length, R = '', k = 0, S, chr, ord;
while(k < len) {
chr = str[k];
if (allowed.indexOf(chr) != -1) {
S = chr;
} else {
ord = str.charCodeAt(k);
if (ord < 256) {
S = '%' + ("00" + ord.toString(16)).toUpperCase().slice(-2);
} else {
S = '%u' + ("0000" + ord.toString(16)).toUpperCase().slice(-4);
}
}
R += S;
k++;
}
return R;
};
})(typeof window == 'undefined' ? global : window);

JS for-loop how to get the first value

I'm sorry for the dumb question. I've been trying to do this for hours now, and i really can't get it to work. So i have a for-loop that loops though some numbers.
But it doesn't take the first value(71990000).
How can this be achieved?
This is what i've got so far:
var minNr = 0000;
var maxNr = 10000;
var prefix = 7199;
function Nummer(min,max)
{
var regex = /^(\d{2})\1$/;
var guld_nr;
for(guld_nr = minNr; guld_nr < maxNr;)
{
if(regex.test(guld_nr))
{
$(".resultat").append(prefix + "" + guld_nr + "<br>");
}
guld_nr++;
}
}
The output is this:
71991010
71991111
71991212
71991313
But i also need the number: 71990000
How can i do that ?

It's because your regex is rejecting the number 0; the first time through the loop, minNr has the numeric value 0 (setting it to 0000 doesn't help; it's just a fancy way of saying 0). The regex expects two digits followed by the same pattern, but what you're giving it is the string '0'.
You could set minNr to be a string instead on the first pass through ('0000'), and this will solve the problem for '0000', but you will miss '0101', '0202', etc. (which will convert to the strings '101', '202', and so on.)
One solution would be to zero pad the string representation of your number. The following function will take any number and left zero pad it to fit a given width:
function zeropad(n, w) {
n = String(n);
while(n.length < w) n = '0' + n;
return n;
}
You can use it to convert minNr for the regex:
regex.test(zeropad(guld_nr, 4))
Also note that Number is a built-in object wrapper for literals in JavaScript (all of the primitives have object wrappers: Number, Boolean, String), and by creating a function called Number, you are occluding this built-in object, which is inadvisable (code that needs to use it will invoke your function instead, which is incompatible and has a different purpose).

Use string:
var minNr = '0000';
It's the start value for the regex test, and you need the four zeroes for that. If it would be a number, then you get only one zero for testing. it would help, if you pad it with leading zeroes.
var minNr = '0000',
maxNr = 10000,
prefix = 7199;
function Nummer(min,max) {
var regex = /^(\d{2})\1$/;
var guld_nr;
for(guld_nr = minNr; guld_nr < maxNr;guld_nr++) {
if(regex.test(guld_nr)) {
document.write(prefix + "" + guld_nr + "<br>");
}
}
}
Nummer(minNr, maxNr);

Numbers don't zero-pad themselves; 0000; // 0
Make a custom zero-pad method for it so you can do zpad(0, 4); // "0000"
function zpad(x, digits) {
var pad = '0';
x = x.toString();
digits -= x.length;
while (digits > 0) {
if (digits & 1) x = pad + x;
pad += pad;
digits >>>= 1;
}
return x;
}
Now adjust Nummer accordingly
function Nummer(min, max, prefix) {
var regex = /^(\d{2})\1$/,
i, str;
prefix = prefix || '';
for(i = min; i < max; ++i) {
str = zpad(i, 4);
if(regex.test(str)) console.log(prefix + str);
}
}
and use
Nummer(minNr, maxNr, '7199');
Side note
Nummer is not constructing an Object, consider camel casing it

You could use arithmetic to do the digit pattern check, and keep the result numerical:
var minNr = 0; // it does not help to put 4 zeroes here.
var maxNr = 10000;
var prefix = 7199;
function Nummer(min,max) {
for (var guld_nr = min; guld_nr < max; guld_nr++) {
if (Math.floor(guld_nr/100) === guld_nr % 100 ) {
$(".resultat").append((prefix * 10000 + guld_nr) + "<br>");
}
}
}
Nummer(minNr, maxNr);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="resultat"></div>
The problem with your code is when the lower numbers are tested against the regular expression, they are implicitly converted to string, and do not get prefixed zeroes, so they fail on the regular expression.
Anyway, the code will be more efficient when sticking to numbers instead of strings, so I would suggest working with numbers all the way up to the point of outputting them in the browser.
Even more efficient is this code:
var minNr = 0; // it does not help to put 4 zeroes here.
var maxNr = 10000;
var prefix = 7199;
function Nummer(min,max) {
var test = Math.floor(min/100)*100 + Math.floor(min/100)%100;
var guld_nr = test < min ? test + 101 : test;
for (; guld_nr < max; guld_nr+=101) {
$(".resultat").append((prefix * 10000 + guld_nr) + "<br>");
}
}
Nummer(minNr, maxNr);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="resultat"></div>

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.