Unicode characters from charcode in javascript for charcodes > 0xFFFF

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!

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]) );