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(['๐']).size, // 4
new Blob(['๐']).size, // 4
new Blob(['๐๐']).size, // 8
new Blob(['๐๐']).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, itโs just an implementation detail that wonโt affect the languageโs 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 ั โ # ยฎ"; // length is 11
const s2 = encodeURIComponent(s); // length is 41
const s3 = unescape(s2); // length is 15 [1-1,a-1,ั-2,โ-3,#-1,ยฎ-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"; // => "โ "
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( 'โค' ); // 4
getStringMemorySize( '๐ ฐ' ); // 8
getStringMemorySize( 'Iโค๐ ฐ' ); // 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) "ฮฉ" (hex: CE A9) and the
third test with three byte character (24bit) "โบ" (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.
Related
I have a string "ใฏใ" and I'm trying to understand how it's represented as bytes.
Number.prototype.toBits = function () {
let str = this.toString(2);
return str.padStart(8, "0");
}
let ja = "ใฏใ";
console.log(ja);
let buf = Buffer.from(ja);
for (const c of buf) {
console.log(c + "=" + c.toBits());
}
produces:
ใฏใ
227=11100011
129=10000001
175=10101111
227=11100011
129=10000001
132=10000100
In the Unicode table, the character "ใฏ" is 306F and the character "ใ" is 3044.
I understand that the leading "1" bit says this is Unicode and that the number of 1s until the next 0 is the number of bytes in Unicode. I don't understand how 306F becomes 11100011 10000001 10101111
The fact that the most-significant bit (MSB) is 1 indicates that it's a UTF-8 multibyte sequence. If the first two bits are 11 then it's the start of a sequence; if 10 it's the continuation of a sequence. Bits of the actual code point are stored in the "unused" portion of both start-bytes and continuation-bytes; as many bytes as are necessary to store the value (and, as indicated by the start-byte).
Notice how it is possible to "drop in anywhere" in the byte-sequence and align yourself to the start of a character: if MSB=0 then it's a single-byte character (ASCII-compatible). If MSBs=10 it's a continuation byte and you should walk-backwards to find the start byte. The start-byte should always be followed by exactly the number of continuation-bytes that it promises. UTF encodings use exactly the number of bytes needed to represent any given Unicode code-point.
According to UTF-8, code points between U+0800 and U+FFFF (which U+306F meets) will be encoded as 3 bytes, spreading their bits across the pattern
1110.... 10...... 10......
The binary representation of 0x306F is 0b11000001101111, which fits in the gaps:
| ....0011 ..000001 ..101111
Together, they form what you are observing:
= 11100011 10000001 10101111
i'm not sure what 0xFF does here...
is it there just to make sure that the binary code is 8bit long or has something to do with the signed/unsigned encoding? ty.
var nBytes = data.length, ui8Data = new Uint8Array(nBytes);
for (var nIdx = 0; nIdx < nBytes; nIdx++) {
ui8Data[nIdx] = data.charCodeAt(nIdx) & 0xff;
}
XHR.send(ui8Data);
You're right with your first guess. It takes only the least significant 8 bits of what's returned by data.charCodeAt.
charCodeAt will return a value in the range of 0..65536. This code truncates that range to 0..255. Effectively, it's taking each 16-bit character in the string, assuming it can fit into 8 bits, and throwing out the upper byte.
[6 years later edit] In the comments, we discovered a few things: you're questioning the code for the MDN polyfill for sendAsBinary. As you came to understand, the least significant byte does come first in little-endian systems, while the most significant byte comes first in big-endian systems.
Given that this is code from MDN, the code certainly does what was intended - by using FileReader.readAsBinaryString, it stores 8bit values into a 16bit holder. If you're worried about data loss, you can tweak the polyfill to extract the other byte using sData.charCodeAt(nIdx) && 0xff00 >> 8.
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'm currently generating UUIDs in Javascript with this function (Create GUID / UUID in JavaScript?):
lucid.uuid = function() {
return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
var r = Math.random()*16|0, v = c == 'x' ? r : (r&0x3|0x8);
return v.toString(16);
});
}
I understand that all the randomness is only coming from Javascript's Math.random() function, and I don't care if it meets an RFC for a UUID. What I want is to pack as much randomness into as few bytes as possible in a Javascript string. The above function gives about 128 bits of randomness. How small of a string (as measured in UTF8 bytes sent over the wire in an HTTP POST) could I fit 128 bits into in Javascript? And how would I generate such a string?
Edit: This string will be part of a JSON object when sent to the server, so characters that need to be escaped in a string are not very helpful.
Here is one potential function I came up with. The seed string is the set of unreserved URL characters (66 of them). I prefix the randomness with about a year's worth of 1-second-resolution timestamp data, which is helpful since the collision space for my particular application is only filled up reasonably slowly over time (only at MOST a few hundred of these generated per second in an extreme case).
uuidDense = function() {
var seed = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_.~';
//Start the UUID with 4 digits of seed from the current date/time in seconds
//(which is almost a year worth of second data).
var seconds = Math.floor((new Date().getTime())/1000);
var ret = seed[seconds % seed.length];
ret += seed[Math.floor(seconds/=seed.length) % seed.length];
ret += seed[Math.floor(seconds/=seed.length) % seed.length];
ret += seed[Math.floor(seconds/=seed.length) % seed.length];
for(var i = 0; i < 8; i++)
ret += seed[Math.random()*seed.length|0];
return ret;
}
Thoughts?
128 bits = 16 bytes -> base64 -> 16*3/2 = will give you string of 24 characters (versus 36 chars that you have)
You also can use base85 for better density but that will require URL encode so you may get even worse results than you have.
Your question is somewhat contradictory. Javascript strings use UCS-2 (fixed 16-bit characters) for their internal representation. However UTF-8 is variable width, but for encoding purposes I believe the most compact form would be to use 1-byte UTF8 characters, which only require the most significant bit be zero. I.e. you could pack 128 bits into 128 * 8/7 = 147 bits.
Converting to bytes and rounding up, you could do this in 19 characters.
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 ๐, but a ํ 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) === '๐'
String.fromCodePoint() seems to do the trick as well. See here.
console.log(String.fromCodePoint(0x1D622, 0x1D623, 0x1D624, 0x1D400));
Output:
๐ข๐ฃ๐ค๐