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I have a javascript string which is about 500K when being sent from the server in UTF-8. How can I tell its size in JavaScript?
I know that JavaScript uses UCS-2, so does that mean 2 bytes per character. However, does it depend on the JavaScript implementation? Or on the page encoding or maybe content-type?
You can use the Blob to get the string size in bytes.
Examples:
console.info(
new Blob(['😂']).size, // 4
new Blob(['👍']).size, // 4
new Blob(['😂👍']).size, // 8
new Blob(['👍😂']).size, // 8
new Blob(['I\'m a string']).size, // 12
// from Premasagar correction of Lauri's answer for
// strings containing lone characters in the surrogate pair range:
// https://stackoverflow.com/a/39488643/6225838
new Blob([String.fromCharCode(55555)]).size, // 3
new Blob([String.fromCharCode(55555, 57000)]).size // 4 (not 6)
);
This function will return the byte size of any UTF-8 string you pass to it.
function byteCount(s) {
return encodeURI(s).split(/%..|./).length - 1;
}
Source
JavaScript engines are free to use UCS-2 or UTF-16 internally. Most engines that I know of use UTF-16, but whatever choice they made, it’s just an implementation detail that won’t affect the language’s characteristics.
The ECMAScript/JavaScript language itself, however, exposes characters according to UCS-2, not UTF-16.
Source
If you're using node.js, there is a simpler solution using buffers :
function getBinarySize(string) {
return Buffer.byteLength(string, 'utf8');
}
There is a npm lib for that : https://www.npmjs.org/package/utf8-binary-cutter (from yours faithfully)
String values are not implementation dependent, according the ECMA-262 3rd Edition Specification, each character represents a single 16-bit unit of UTF-16 text:
4.3.16 String Value
A string value is a member of the type String and is a
finite ordered sequence of zero or
more 16-bit unsigned integer values.
NOTE Although each value usually
represents a single 16-bit unit of
UTF-16 text, the language does not
place any restrictions or requirements
on the values except that they be
16-bit unsigned integers.
These are 3 ways I use:
TextEncoder
new TextEncoder().encode("myString").length
Blob
new Blob(["myString"]).size
Buffer
Buffer.byteLength("myString", 'utf8')
Try this combination with using unescape js function:
const byteAmount = unescape(encodeURIComponent(yourString)).length
Full encode proccess example:
const s = "1 a ф № # ®"; // length is 11
const s2 = encodeURIComponent(s); // length is 41
const s3 = unescape(s2); // length is 15 [1-1,a-1,ф-2,№-3,#-1,®-2]
const s4 = escape(s3); // length is 39
const s5 = decodeURIComponent(s4); // length is 11
Note that if you're targeting node.js you can use Buffer.from(string).length:
var str = "\u2620"; // => "☠"
str.length; // => 1 (character)
Buffer.from(str).length // => 3 (bytes)
The size of a JavaScript string is
Pre-ES6: 2 bytes per character
ES6 and later: 2 bytes per character,
or 5 or more bytes per character
Pre-ES6
Always 2 bytes per character. UTF-16 is not allowed because the spec says "values must be 16-bit unsigned integers". Since UTF-16 strings can use 3 or 4 byte characters, it would violate 2 byte requirement. Crucially, while UTF-16 cannot be fully supported, the standard does require that the two byte characters used are valid UTF-16 characters. In other words, Pre-ES6 JavaScript strings support a subset of UTF-16 characters.
ES6 and later
2 bytes per character, or 5 or more bytes per character. The additional sizes come into play because ES6 (ECMAScript 6) adds support for Unicode code point escapes. Using a unicode escape looks like this: \u{1D306}
Practical notes
This doesn't relate to the internal implemention of a particular engine. For
example, some engines use data structures and libraries with full
UTF-16 support, but what they provide externally doesn't have to be
full UTF-16 support. Also an engine may provide external UTF-16
support as well but is not mandated to do so.
For ES6, practically speaking characters will never be more than 5
bytes long (2 bytes for the escape point + 3 bytes for the Unicode
code point) because the latest version of Unicode only has 136,755
possible characters, which fits easily into 3 bytes. However this is
technically not limited by the standard so in principal a single
character could use say, 4 bytes for the code point and 6 bytes
total.
Most of the code examples here for calculating byte size don't seem to take into account ES6 Unicode code point escapes, so the results could be incorrect in some cases.
UTF-8 encodes characters using 1 to 4 bytes per code point. As CMS pointed out in the accepted answer, JavaScript will store each character internally using 16 bits (2 bytes).
If you parse each character in the string via a loop and count the number of bytes used per code point, and then multiply the total count by 2, you should have JavaScript's memory usage in bytes for that UTF-8 encoded string. Perhaps something like this:
getStringMemorySize = function( _string ) {
"use strict";
var codePoint
, accum = 0
;
for( var stringIndex = 0, endOfString = _string.length; stringIndex < endOfString; stringIndex++ ) {
codePoint = _string.charCodeAt( stringIndex );
if( codePoint < 0x100 ) {
accum += 1;
continue;
}
if( codePoint < 0x10000 ) {
accum += 2;
continue;
}
if( codePoint < 0x1000000 ) {
accum += 3;
} else {
accum += 4;
}
}
return accum * 2;
}
Examples:
getStringMemorySize( 'I' ); // 2
getStringMemorySize( '❤' ); // 4
getStringMemorySize( '𠀰' ); // 8
getStringMemorySize( 'I❤𠀰' ); // 14
The answer from Lauri Oherd works well for most strings seen in the wild, but will fail if the string contains lone characters in the surrogate pair range, 0xD800 to 0xDFFF. E.g.
byteCount(String.fromCharCode(55555))
// URIError: URI malformed
This longer function should handle all strings:
function bytes (str) {
var bytes=0, len=str.length, codePoint, next, i;
for (i=0; i < len; i++) {
codePoint = str.charCodeAt(i);
// Lone surrogates cannot be passed to encodeURI
if (codePoint >= 0xD800 && codePoint < 0xE000) {
if (codePoint < 0xDC00 && i + 1 < len) {
next = str.charCodeAt(i + 1);
if (next >= 0xDC00 && next < 0xE000) {
bytes += 4;
i++;
continue;
}
}
}
bytes += (codePoint < 0x80 ? 1 : (codePoint < 0x800 ? 2 : 3));
}
return bytes;
}
E.g.
bytes(String.fromCharCode(55555))
// 3
It will correctly calculate the size for strings containing surrogate pairs:
bytes(String.fromCharCode(55555, 57000))
// 4 (not 6)
The results can be compared with Node's built-in function Buffer.byteLength:
Buffer.byteLength(String.fromCharCode(55555), 'utf8')
// 3
Buffer.byteLength(String.fromCharCode(55555, 57000), 'utf8')
// 4 (not 6)
A single element in a JavaScript String is considered to be a single UTF-16 code unit. That is to say, Strings characters are stored in 16-bit (1 code unit), and 16-bit is equal to 2 bytes (8-bit = 1 byte).
The charCodeAt() method can be used to return an integer between 0 and 65535 representing the UTF-16 code unit at the given index.
The codePointAt() can be used to return the entire code point value for Unicode characters, e.g. UTF-32.
When a UTF-16 character can't be represented in a single 16-bit code unit, it will have a surrogate pair and therefore use two code units( 2 x 16-bit = 4 bytes)
See Unicode encodings for different encodings and their code ranges.
The Blob interface's size property returns the size of the Blob or File in bytes.
const getStringSize = (s) => new Blob([s]).size;
I'm working with an embedded version of the V8 Engine.
I've tested a single string. Pushing each step 1000 characters. UTF-8.
First test with single byte (8bit, ANSI) Character "A" (hex: 41).
Second test with two byte character (16bit) "Ω" (hex: CE A9) and the
third test with three byte character (24bit) "☺" (hex: E2 98 BA).
In all three cases the device prints out of memory at
888 000 characters and using ca. 26 348 kb in RAM.
Result: The characters are not dynamically stored. And not with only 16bit. - Ok, perhaps only for my case (Embedded 128 MB RAM Device, V8 Engine C++/QT) - The character encoding has nothing to do with the size in ram of the javascript engine. E.g. encodingURI, etc. is only useful for highlevel data transmission and storage.
Embedded or not, fact is that the characters are not only stored in 16bit.
Unfortunally I've no 100% answer, what Javascript do at low level area.
Btw. I've tested the same (first test above) with an array of character "A".
Pushed 1000 items every step. (Exactly the same test. Just replaced string to array) And the system bringt out of memory (wanted) after 10 416 KB using and array length of 1 337 000.
So, the javascript engine is not simple restricted. It's a kind more complex.
You can try this:
var b = str.match(/[^\x00-\xff]/g);
return (str.length + (!b ? 0: b.length));
It worked for me.
I'm getting too confused. Why do code points from U+D800 to U+DBFF encode as a single (2 bytes) String element, when using the ECMAScript 6 native Unicode helpers?
I'm not asking how JavaScript/ECMAScript encodes Strings natively, I'm asking about an extra functionality to encode UTF-16 that makes use of UCS-2.
var str1 = '\u{D800}';
var str2 = String.fromCodePoint(0xD800);
console.log(
str1.length, str1.charCodeAt(0), str1.charCodeAt(1)
);
console.log(
str2.length, str2.charCodeAt(0), str2.charCodeAt(1)
);
Re-TL;DR: I want to know why the above approaches return a string of length 1. Shouldn't U+D800 generate a 2 length string, since my browser's ES6 implementation incorporates UCS-2 encoding in strings, which uses 2 bytes for each character code?
Both of these approaches return a one-element String for the U+D800 code point (char code: 55296, same as 0xD800). But for code points bigger than U+FFFF each one returns a two-element String, the lead and trail. lead would be a number between U+D800 and U+DBFF, and trail I'm not sure about, I only know it helps changing the result code point. For me the return value doesn't make sense, it represents a lead without trail. Am I understanding something wrong?
I think your confusion is about how Unicode encodings work in general, so let me try to explain.
Unicode itself just specifies a list of characters, called "code points", in a particular order. It doesn't tell you how to convert those to bits, it just gives them all a number between 0 and 1114111 (in hexadecimal, 0x10FFFF). There are several different ways these numbers from U+0 to U+10FFFF can be represented as bits.
In an earlier version, it was expected that a range of 0 to 65535 (0xFFFF) would be enough. This can be naturally represented in 16 bits, using the same convention as an unsigned integer. This was the original way of storing Unicode, and is now known as UCS-2. To store a single code point, you reserve 16 bits of memory.
Later, it was decided that this range was not large enough; this meant that there were code points higher than 65535, which you can't represent in a 16-bit piece of memory. UTF-16 was invented as a clever way of storing these higher code points. It works by saying "if you look at a 16-bit piece of memory, and it's a number between 0xD800 and 0xDBF (a "low surrogate"), then you need to look at the next 16 bits of memory as well". Any piece of code which is performing this extra check is processing its data as UTF-16, and not UCS-2.
It's important to understand that the memory itself doesn't "know" which encoding it's in, the difference between UCS-2 and UTF-16 is how you interpret that memory. When you write a piece of software, you have to choose which interpretation you're going to use.
Now, onto Javascript...
Javascript handles input and output of strings by interpreting its internal representation as UTF-16. That's great, it means that you can type in and display the famous 💩 character, which can't be stored in one 16-bit piece of memory.
The problem is that most of the built in string functions actually handle the data as UCS-2 - that is, they look at 16 bits at a time, and don't care if what they see is a special "surrogate". The function you used, charCodeAt(), is an example of this: it reads 16 bits out of memory, and gives them to you as a number between 0 and 65535. If you feed it 💩, it will just give you back the first 16 bits; ask it for the next "character" after, and it will give you the second 16 bits (which will be a "high surrogate", between 0xDC00 and 0xDFFF).
In ECMAScript 6 (2015), a new function was added: codePointAt(). Instead of just looking at 16 bits and giving them to you, this function checks if they represent one of the UTF-16 surrogate code units, and if so, looks for the "other half" - so it gives you a number between 0 and 1114111. If you feed it 💩, it will correctly give you 128169.
var poop = '💩';
console.log('Treat it as UCS-2, two 16-bit numbers: ' + poop.charCodeAt(0) + ' and ' + poop.charCodeAt(1));
console.log('Treat it as UTF-16, one value cleverly encoded in 32 bits: ' + poop.codePointAt(0));
// The surrogates are 55357 and 56489, which encode 128169 as follows:
// 0x010000 + ((55357 - 0xD800) << 10) + (56489 - 0xDC00) = 128169
Your edited question now asks this:
I want to know why the above approaches return a string of length 1. Shouldn't U+D800 generate a 2 length string?
The hexadecimal value D800 is 55296 in decimal, which is less than 65536, so given everything I've said above, this fits fine in 16 bits of memory. So if we ask charCodeAt to read 16 bits of memory, and it finds that number there, it's not going to have a problem.
Similarly, the .length property measures how many sets of 16 bits there are in the string. Since this string is stored in 16 bits of memory, there is no reason to expect any length other than 1.
The only unusual thing about this number is that in Unicode, that value is reserved - there isn't, and never will be, a character U+D800. That's because it's one of the magic numbers that tells a UTF-16 algorithm "this is only half a character". So a possible behaviour would be for any attempt to create this string to simply be an error - like opening a pair of brackets that you never close, it's unbalanced, incomplete.
The only way you could end up with a string of length 2 is if the engine somehow guessed what the second half should be; but how would it know? There are 1024 possibilities, from 0xDC00 to 0xDFFF, which could be plugged into the formula I show above. So it doesn't guess, and since it doesn't error, the string you get is 16 bits long.
Of course, you can supply the matching halves, and codePointAt will interpret them for you.
// Set up two 16-bit pieces of memory
var high=String.fromCharCode(55357), low=String.fromCharCode(56489);
// Note: String.fromCodePoint will give the same answer
// Glue them together (this + is string concatenation, not number addition)
var poop = high + low;
// Read out the memory as UTF-16
console.log(poop);
console.log(poop.codePointAt(0));
Well, it does this because the specification says it has to:
http://www.ecma-international.org/ecma-262/6.0/#sec-string.fromcodepoint
http://www.ecma-international.org/ecma-262/6.0/#sec-utf16encoding
Together these two say that if an argument is < 0 or > 0x10FFFF, a RangeError is thrown, but otherwise any codepoint <= 65535 is incorporated into the result string as-is.
As for why things are specified this way, I don't know. It seems like JavaScript doesn't really support Unicode, only UCS-2.
Unicode.org has the following to say on the matter:
http://www.unicode.org/faq/utf_bom.html#utf16-2
Q: What are surrogates?
A: Surrogates are code points from two special ranges of Unicode values, reserved for use as the leading, and trailing values of paired code units in UTF-16. Leading, also called high, surrogates are from D80016 to DBFF16, and trailing, or low, surrogates are from DC0016 to DFFF16. They are called surrogates, since they do not represent characters directly, but only as a pair.
http://www.unicode.org/faq/utf_bom.html#utf16-7
Q: Are there any 16-bit values that are invalid?
A: Unpaired surrogates are invalid in UTFs. These include any value in the range D80016 to DBFF16 not followed by a value in the range DC0016 to DFFF16, or any value in the range DC0016 to DFFF16 not preceded by a value in the range D80016 to DBFF16.
Therefore the result of String.fromCodePoint is not always valid UTF-16 because it can emit unpaired surrogates.
What is the difference between String.prototype.codePointAt() and String.prototype.charCodeAt() in JavaScript?
'A'.codePointAt(); // 65
'A'.charCodeAt(); // 65
From the MDN page on charCodeAt:
The charCodeAt() method returns an integer between 0 and 65535 representing the UTF-16 code unit at the given index.
The UTF-16 code unit matches the Unicode code point for code points which can be represented in a single UTF-16 code unit. If the Unicode code point cannot be represented in a single UTF-16 code unit (because its value is greater than 0xFFFF) then the code unit returned will be the first part of a surrogate pair for the code point. If you want the entire code point value, use codePointAt().
TLDR;
charCodeAt() is UTF-16
codePointAt() is Unicode.
To add a few for the ToxicTeacakes's answer, here is another example to help you know the difference:
"𠮷".charCodeAt(0).toString(16);//d842
"𠮷".charCodeAt(1).toString(16);//dfb7
"𠮷".codePointAt(0);//20bb7
"𠮷".codePointAt(1);//dfb7
console.log("\ud842\udfb7");//𠮷, an example of hexadecimal digits
console.log("\u20bb7\udfb7");//₻7�
console.log("\u{20bb7}");//𠮷 an unicode code point escapes the "\ud842\udfb7"
The following is the info about javascript string literals:
"\uXXXX"
The Unicode character specified by the four hexadecimal digits
XXXX. For example, \u00A9 is the Unicode sequence for the copyright
symbol.
"\u{XXXXX}"
Unicode code point
escapes. For example, \u{2F804} is the same as the simple Unicode
escapes \uD87E\uDC04.
see also msdn
Example in JS
On The example with strings and emojis, I am going to illustrate how things could go wrong when you do not know that some of the characters could consist of 2 code units. Some of the characters take up more than one code unit. Consider using codePointAt() over charCodeAt() or use the first one if you are sure that your characters lie in of between 0 and 65535 (216)
more about code units here
// charCodeAt() is UTF-16
// codePointAt() is Unicode
/* UTF-16 is generally considered a bad idea today */
const strings = ["o", "four", "to"];
const emojis = ["🐎", "👟"];
function printItemsLength(arr) {
for (const item of arr) {
console.log(item, item.length);
}
}
printItemsLength(strings);
console.log('================================');
printItemsLength(emojis);
console.log('================================');
console.log("i.charCodeAt(0)", "i".charCodeAt(0)); // 105
console.log("i.charCodeAt(1)", "i".charCodeAt(1)); // 105
console.log("i.codePointAt(0)", "i".codePointAt(0)); // 105
console.log('=============EMOJIS=============');
// getting the decimal (dec) by which you can find them
console.log('===========charCodeAt===========');
// "surrogate pair"
console.log(emojis[0] + '.charCodeAt(0)', emojis[0].charCodeAt(0)); // only half-character - 55357
console.log(emojis[0] + '.charCodeAt(1)', emojis[0].charCodeAt(1)); // only half-character - 55357
console.log('===========codePointAt===========');
console.log(emojis[0] + '.codePointAt(0)', emojis[0].codePointAt(0)); // 128014
console.log('===========charCodeAt===========');
// "surrogate pair"
console.log(emojis[1] + '.charCodeAt(0)', emojis[1].charCodeAt(0)); // only half-character - 55357
console.log(emojis[1] + '.charCodeAt(1)', emojis[1].charCodeAt(1)); // only half-character - 55357
console.log('===========codePointAt===========');
// full-character
console.log(emojis[1] + '.codePointAt(0)', emojis[1].codePointAt(0)); // 128095
console.log(emojis[1] + '.codePointAt(1)', emojis[1].codePointAt(1)); // will return lower surragate (non-displayable character)
// to find this emojis have a look here: https://www.w3schools.com/charsets/ref_emoji.asp
as someone may have noticed I have tried to convert back from charcode to the emoji, and it did not work on one of the symbols (that is because it is not in range of UTF-16
Introduction to Unicode and UTF-16
please skip this section if you already familiar with it
Unicode – is a set of characters used around the world; UTF-16 -
00000000 00100100 for "$" (one 16-bits);11011000 01010010 11011111
01100010 for "𤭢" (two 16-bits)
read more
"surrogate pair" characters are emoji and some letters that consist of more than 1 character as it is explained here
The term "surrogate pair" refers to a means of encoding Unicode
characters with high code-points in the UTF-16 encoding scheme. In the
Unicode character encoding, characters are mapped to values between
0x0 and 0x10FFFF.
read more
Unicode - It assigns every character a unique number called a code point.
Differentiating charCodeAt() from codePointAt()
charCodeAt(pos) returns code a code unit (not a full character).
If you need a character (that could be either one or two code units), you can use codePointAt(pos) to get its code.
charCodeAt() - returns an integer between 0 and 65535 representing the UTF-16 code unit at the given index link
codePointAt() - returns a non-negative integer that is the Unicode code point value at the given position link
where pos is the index of the character you want to check.
Quote from the book:
UTF-16 is generally considered a bad idea today. It seems almost
intentionally designed to invite mistakes. It’s easy to write programs
that pretend code units and characters are the same things.
read more
jsfiddle sandbox
Sources:
What is Unicode, UTF-8, UTF-16?
Marijn Haverbeke Eloquent JavaScript, 3rd Edition: A Modern Introduction to Programming [Text] – City(not-specified) : No Starch Press, 2018 – 447 p. can be found here
What is "surrogate pair"
to find this emojis have a look w3schools.com/charsets/ref_emoji
Chapter 5, p. 91 => Strings and character codes
How to count bits of the string in JavaScript?
For example how many bits long is the string 0000xfe-kemZlF4IlEgljDF_4df:1102pwrq7?
The string provided ("0000xfe-kemZlF4IlEgljDF_4df:1102pwrq7") would be:
length * 2 * 8
bits long, or 592 bits.
This is because each char in a string is treated as a 16-bit unsigned value, at least in the most common mainstream implementation. The details of this can probably be discussed, but you mention in the comments that it is for security purposes -
So assuming you are giving ASCII characters (0-127) or UTF-8 (0-255) you can use the TextEncoder object to make sure you provide enough chars to produce 128 bits. Just be careful with Latin-1 chars in UTF-8 as the encoder may project them to the UTF-16 equivalent meaning it will produce 2 bytes for it instead of just one.
If you use a plain JavaScript string to hold ASCII characters you will have half the positions represented as 0's which reduce the security significantly, so an encoding from UTF-16/UCS-2 to ASCII or UTF-8 is required.
To use TextEncoder you simply provide a string representing 16 characters, at this point 256 bits (16x16) but where each char is within the ASCII/UTF-8 value range. After encoding, unless some special chars where used, the binary buffer as typed array should represent 128 bits (16x8).
Example
if (!("TextEncoder" in window)) alert("Sorry, no TextEncoder in this browser...");
else {
btn.onclick = function() {
var s = txt.value;
if (s.length !== 16) {
alert("Need 16 chars. " + (16 - s.length) + " to go...");
return
}
var encoder = new TextEncoder("ASCII"); // or use UTF-8
var bytes = encoder.encode(s);
console.log(bytes);
if (bytes.byteLength === 16) alert("OK, got 128 bits");
else alert("Oops, got " + (bytes.byteLength * 8) + " bits.");
};
}
<label>Enter 16 ASCII chars: <input id=txt maxlength=16></label>
<button id=btn>Convert</button>
An alternative to TextEncoder if using older browsers is to manually iterate over the string and extract and mask each char to build a binary array from that.
Can you copy the string into a buffer and then check the length of the buffer?
var str = ' ... ';
var buf = new Buffer(str);
console.log(buf.length);
If, as you say, you just need to make sure the given value is at least 128 bit, then you're probably passing this string to something that will be converting the string to some byte representation. How the string is converted to bytes depends on how it's encoded.
The sample string you gave us contains ASCII-range characters. If the string is encoded as ASCII, then it's 8 bits per character. If the string was encoded as UTF-8, then it would be 8 bits per character, but if the string could contain larger character values than the sample you provided, then it may be more than 8 bits per character depending on the character. If it's encoded as UTF-16, then each character is a minimum of 16 bits, but could be more depending on the character. If it's encoded as USC-2, then it would always be 16 bits per character.
We don't know where this requirement is coming from and how the system requiring this string uses it. If the system uses a fixed number of bits per character, then this is as straightforward as taking the length of the string and multiplying by the appropriate number. If it's not that straightforward, then you would need to encode the string using the proper encoding, most likely to a byte array, then multiply 8 * the number of bytes to get the number of bits.
I need to get a string / char from a unicode charcode and finally put it into a DOM TextNode to add into an HTML page using client side JavaScript.
Currently, I am doing:
String.fromCharCode(parseInt(charcode, 16));
where charcode is a hex string containing the charcode, e.g. "1D400". The unicode character which should be returned is 𝐀, but a 퐀 is returned! Characters in the 16 bit range (0000 ... FFFF) are returned as expected.
Any explanation and / or proposals for correction?
Thanks in advance!
String.fromCharCode can only handle code points in the BMP (i.e. up to U+FFFF). To handle higher code points, this function from Mozilla Developer Network may be used to return the surrogate pair representation:
function fixedFromCharCode (codePt) {
if (codePt > 0xFFFF) {
codePt -= 0x10000;
return String.fromCharCode(0xD800 + (codePt >> 10), 0xDC00 + (codePt & 0x3FF));
} else {
return String.fromCharCode(codePt);
}
}
The problem is that characters in JavaScript are (mostly) UCS-2 encoded but can represent a character outside the Basic Multilingual Plane in JavaScript as a UTF-16 surrogate pair.
The following function is adapted from Converting punycode with dash character to Unicode:
function utf16Encode(input) {
var output = [], i = 0, len = input.length, value;
while (i < len) {
value = input[i++];
if ( (value & 0xF800) === 0xD800 ) {
throw new RangeError("UTF-16(encode): Illegal UTF-16 value");
}
if (value > 0xFFFF) {
value -= 0x10000;
output.push(String.fromCharCode(((value >>>10) & 0x3FF) | 0xD800));
value = 0xDC00 | (value & 0x3FF);
}
output.push(String.fromCharCode(value));
}
return output.join("");
}
alert( utf16Encode([0x1D400]) );
Section 8.4 of the EcmaScript language spec says
When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.
So you need to encode supplemental code-points as pairs of UTF-16 code units.
The article "Supplementary Characters in the Java Platform" gives a good description of how to do this.
UTF-16 uses sequences of one or two unsigned 16-bit code units to encode Unicode code points. Values U+0000 to U+FFFF are encoded in one 16-bit unit with the same value. Supplementary characters are encoded in two code units, the first from the high-surrogates range (U+D800 to U+DBFF), the second from the low-surrogates range (U+DC00 to U+DFFF). This may seem similar in concept to multi-byte encodings, but there is an important difference: The values U+D800 to U+DFFF are reserved for use in UTF-16; no characters are assigned to them as code points. This means, software can tell for each individual code unit in a string whether it represents a one-unit character or whether it is the first or second unit of a two-unit character. This is a significant improvement over some traditional multi-byte character encodings, where the byte value 0x41 could mean the letter "A" or be the second byte of a two-byte character.
The following table shows the different representations of a few characters in comparison:
code points / UTF-16 code units
U+0041 / 0041
U+00DF / 00DF
U+6771 / 6771
U+10400 / D801 DC00
Once you know the UTF-16 code units, you can create a string using the javascript function String.fromCharCode:
String.fromCharCode(0xd801, 0xdc00) === '𐐀'
String.fromCodePoint() seems to do the trick as well. See here.
console.log(String.fromCodePoint(0x1D622, 0x1D623, 0x1D624, 0x1D400));
Output:
𝘢𝘣𝘤𝐀