How many bytes in a JavaScript string?

Question

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?

Answer 1

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.

Answer 2

Try this combination with using unescape js function:

var byteAmount = unescape(encodeURIComponent(yourString)).length

Full encode proccess example:

    var s  = "1 a ф № @ ®"; //length is 11
    var s2 = encodeURIComponent(s); //length is 41
    var s3 = unescape(s2); //length is 15 [1-1,a-1,ф-2,№-3,@-1,®-2]
    var s4 = escape(s3); //length is 39
    var s5 = decodeURIComponent(s4); //length is 11

See aditional screen http://dl.dropbox.com/u/2086213/%3Dcoding%3D/js_utf_byte_length.png (I am a new user, so I can't use the img tag)

Answer 3

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

Answer 4

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.

Answer 5

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( '                                                                    

                                                        
            

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Answer 6

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.