DataBuffer.js

writeUTF8(str) {
        let utf8 = []
        let utflen = str.length; // optimized for ASCII

        for (let i = 0; i < str.length; i++) {
            let c = str.charAt(i);
            if (c >= 0x80 || c === 0)
                utflen += (c >= 0x800) ? 2 : 1;
        }

        utf8.push(rshift(utflen, 8) & 0xFF)
        utf8.push(rshift(utflen, 0) & 0xFF)

        for (let i = 0; i < str.length; i++) {
            let charcode = str.charCodeAt(i)
            if (charcode < 0x80) utf8.push(charcode)
            else if (charcode < 0x800) {
                utf8.push(0xC0 | (charcode >> 6), 0x80 | (charcode & 0x3f))
            }
            else if (charcode < 0xd800 || charcode >= 0xe000) {
                utf8.push(0xE0 | (charcode >> 12),
                    0x80 | ((charcode>>6) & 0x3f),
                    0x80 | (charcode & 0x3f))
            }
            // surrogate pair
            else {
                i++
                charcode = ((charcode&0x3ff)<<10)|(str.charCodeAt(i)&0x3ff)
                utf8.push(0xf0 | (charcode >>18),
                    0x80 | ((charcode>>12) & 0x3f),
                    0x80 | ((charcode>>6) & 0x3f),
                    0x80 | (charcode & 0x3f))
            }
        }
        this.writeByteArray(utf8)
    }