JavaScript strings outside of the BMP

BMP being Basic Multilingual Plane

According to JavaScript: the Good Parts:

JavaScript was built at a time when Unicode was a 16-bit character set, so all characters in JavaScript are 16 bits wide.

This leads me to believe that JavaScript uses UCS-2 (not UTF-16!) and can only handle characters up to U+FFFF.

Further investigation confirms this:

> String.fromCharCode(0x20001);

The fromCharCode method seems to only use the lowest 16 bits when returning the Unicode character. Trying to get U+20001 (CJK unified ideograph 20001) instead returns U+0001.

Question: is it at all possible to handle post-BMP characters in JavaScript?


2011-07-31: slide twelve from Unicode Support Shootout: The Good, The Bad, & the (mostly) Ugly covers issues related to this quite well:

Answers:

Answer

Depends what you mean by ‘support’. You can certainly put non-UCS-2 characters in a JS string using surrogates, and browsers will display them if they can.

But, each item in a JS string is a separate UTF-16 code unit. There is no language-level support for handling full characters: all the standard String members (length, split, slice etc) all deal with code units not characters, so will quite happily split surrogate pairs or hold invalid surrogate sequences.

If you want surrogate-aware methods, I'm afraid you're going to have to start writing them yourself! For example:

String.prototype.getCodePointLength= function() {
    return this.length-this.split(/[\uD800-\uDBFF][\uDC00-\uDFFF]/g).length+1;
};

String.fromCodePoint= function() {
    var chars= Array.prototype.slice.call(arguments);
    for (var i= chars.length; i-->0;) {
        var n = chars[i]-0x10000;
        if (n>=0)
            chars.splice(i, 1, 0xD800+(n>>10), 0xDC00+(n&0x3FF));
    }
    return String.fromCharCode.apply(null, chars);
};
Answer

I came to the same conclusion as bobince. If you want to work with strings containing unicode characters outside of the BMP, you have to reimplement javascript's String methods. This is because javascript counts characters as each 16-bit code value. Symbols outside of the BMP need two code values to be represented. You therefore run into a case where some symbols count as two characters and some count only as one.

I've reimplemented the following methods to treat each unicode code point as a single character: .length, .charCodeAt, .fromCharCode, .charAt, .indexOf, .lastIndexOf, .splice, and .split.

You can check it out on jsfiddle: http://jsfiddle.net/Y89Du/

Here's the code without comments. I tested it, but it may still have errors. Comments are welcome.

if (!String.prototype.ucLength) {
    String.prototype.ucLength = function() {
        // this solution was taken from 
        // http://stackoverflow.com/questions/3744721/javascript-strings-outside-of-the-bmp
        return this.length - this.split(/[\uD800-\uDBFF][\uDC00-\uDFFF]/g).length + 1;
    };
}

if (!String.prototype.codePointAt) {
    String.prototype.codePointAt = function (ucPos) {
        if (isNaN(ucPos)){
            ucPos = 0;
        }
        var str = String(this);
        var codePoint = null;
        var pairFound = false;
        var ucIndex = -1;
        var i = 0;  
        while (i < str.length){
            ucIndex += 1;
            var code = str.charCodeAt(i);
            var next = str.charCodeAt(i + 1);
            pairFound = (0xD800 <= code && code <= 0xDBFF && 0xDC00 <= next && next <= 0xDFFF);
            if (ucIndex == ucPos){
                codePoint = pairFound ? ((code - 0xD800) * 0x400) + (next - 0xDC00) + 0x10000 : code;
                break;
            } else{
                i += pairFound ? 2 : 1;
            }
        }
        return codePoint;
    };
}

if (!String.fromCodePoint) {
    String.fromCodePoint = function () {
        var strChars = [], codePoint, offset, codeValues, i;
        for (i = 0; i < arguments.length; ++i) {
            codePoint = arguments[i];
            offset = codePoint - 0x10000;
            if (codePoint > 0xFFFF){
                codeValues = [0xD800 + (offset >> 10), 0xDC00 + (offset & 0x3FF)];
            } else{
                codeValues = [codePoint];
            }
            strChars.push(String.fromCharCode.apply(null, codeValues));
        }
        return strChars.join("");
    };
}

if (!String.prototype.ucCharAt) {
    String.prototype.ucCharAt = function (ucIndex) {
        var str = String(this);
        var codePoint = str.codePointAt(ucIndex);
        var ucChar = String.fromCodePoint(codePoint);
        return ucChar;
    };
}

if (!String.prototype.ucIndexOf) {
    String.prototype.ucIndexOf = function (searchStr, ucStart) {
        if (isNaN(ucStart)){
            ucStart = 0;
        }
        if (ucStart < 0){
            ucStart = 0;
        }
        var str = String(this);
        var strUCLength = str.ucLength();
        searchStr = String(searchStr);
        var ucSearchLength = searchStr.ucLength();
        var i = ucStart;
        while (i < strUCLength){
            var ucSlice = str.ucSlice(i,i+ucSearchLength);
            if (ucSlice == searchStr){
                return i;
            }
            i++;
        }
        return -1;
    };
}

if (!String.prototype.ucLastIndexOf) {
    String.prototype.ucLastIndexOf = function (searchStr, ucStart) {
        var str = String(this);
        var strUCLength = str.ucLength();
        if (isNaN(ucStart)){
            ucStart = strUCLength - 1;
        }
        if (ucStart >= strUCLength){
            ucStart = strUCLength - 1;
        }
        searchStr = String(searchStr);
        var ucSearchLength = searchStr.ucLength();
        var i = ucStart;
        while (i >= 0){
            var ucSlice = str.ucSlice(i,i+ucSearchLength);
            if (ucSlice == searchStr){
                return i;
            }
            i--;
        }
        return -1;
    };
}

if (!String.prototype.ucSlice) {
    String.prototype.ucSlice = function (ucStart, ucStop) {
        var str = String(this);
        var strUCLength = str.ucLength();
        if (isNaN(ucStart)){
            ucStart = 0;
        }
        if (ucStart < 0){
            ucStart = strUCLength + ucStart;
            if (ucStart < 0){ ucStart = 0;}
        }
        if (typeof(ucStop) == 'undefined'){
            ucStop = strUCLength - 1;
        }
        if (ucStop < 0){
            ucStop = strUCLength + ucStop;
            if (ucStop < 0){ ucStop = 0;}
        }
        var ucChars = [];
        var i = ucStart;
        while (i < ucStop){
            ucChars.push(str.ucCharAt(i));
            i++;
        }
        return ucChars.join("");
    };
}

if (!String.prototype.ucSplit) {
    String.prototype.ucSplit = function (delimeter, limit) {
        var str = String(this);
        var strUCLength = str.ucLength();
        var ucChars = [];
        if (delimeter == ''){
            for (var i = 0; i < strUCLength; i++){
                ucChars.push(str.ucCharAt(i));
            }
            ucChars = ucChars.slice(0, 0 + limit);
        } else{
            ucChars = str.split(delimeter, limit);
        }
        return ucChars;
    };
}
Answer

More recent JavaScript engines have String.fromCodePoint.

const ideograph = String.fromCodePoint( 0x20001 ); // outside the BMP

Also a code-point iterator, which gets you the code-point length.

function countCodePoints( str )
{
    const i = str[Symbol.iterator]();
    let count = 0;
    while( !i.next().done ) ++count;
    return count;
}

console.log( ideograph.length ); // gives '2'
console.log( countCodePoints(ideograph) ); // '1'
Answer

Yes, you can. Although support to non-BMP characters directly in source documents is optional according to the ECMAScript standard, modern browsers let you use them. Naturally, the document encoding must be properly declared, and for most practical purposes you would need to use the UTF-8 encoding. Moreover, you need an editor that can handle UTF-8, and you need some input method(s); see e.g. my Full Unicode Input utility.

Using suitable tools and settings, you can write var foo = '????'.

The non-BMP characters will be internally represented as surrogate pairs, so each non-BMP character counts as 2 in the string length.

Answer

Using for (c of this) instruction, one can make various computations on a string that contains non-BMP characters. For instance, to compute the string length, and to get the nth character of the string:

String.prototype.magicLength = function()
{
    var c, k;
    k = 0;
    for (c of this) // iterate each char of this
    {
        k++;
    }
    return k;
}

String.prototype.magicCharAt = function(n)
{
    var c, k;
    k = 0;
    for (c of this) // iterate each char of this
    {
        if (k == n) return c + "";
        k++;
    }
    return "";
}

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