Duplicate a StringDecoder

duplicateStringDecoder.js

function duplicateStringDecoder(decoder) {
    let r = new StringDecoder(decoder.encoding);
    let partialCharBytes = decoder.lastTotal - decoder.lastNeed;

    if (partialCharBytes > 0) {
        r.write(decoder.lastChar.slice(0, partialCharBytes));
    }

    return r;
}