Seems to work... Not nearly as fast as node-bencode

bencodeDecode.js

const MODE_NONE = 0,
      MODE_BYTESTRING = 1,
      MODE_INT = 2,
      MODE_LIST = 3,
      MODE_DICT = 4;

function getInt(buf) {
    let n = 0;
    for (let i = 0; i < buf.length; ++i) {
        n += (buf[buf.length - 1 - i] - 48) * 10 ** i;
    }
    return n;
}

function _decode(data) {
    const items = [];
    
    let buf = [];
    const l = data.length;
    let i = 0;
    let mode = MODE_NONE;
    let bytesLength = 0;
    let bytesRead = 0;
    let bytesBuf;

    while (i < l) {
        const byte = data[i];
        if (mode === MODE_NONE) {
            if (byte === 101) { // e
                i += 1;
                break;
            } else if (byte === 105) { // i
                mode = MODE_INT;
            } else if (byte === 108) { // l
                mode = MODE_LIST;
                const [listItems, listSize] = _decode(data.slice(i + 1));
                items.push(listItems);
                i += listSize + 1;
                mode = MODE_NONE;
                continue;
            } else if (byte === 100) { // d
                mode = MODE_DICT;
                const [dictItems, dictSize] = _decode(data.slice(i + 1));
                const dict = {};
                for (let j = 0; j < dictItems.length; j += 2) {
                    dict[dictItems[j]] = dictItems[j + 1];
                }
                items.push(dict);
                i += dictSize + 1;
                mode = MODE_NONE;
                continue;
            } else if (byte >= 48 && byte <= 57) { // 0-9
                bytesLength *= 10;
                bytesLength += byte - 48;
            } else if (byte === 58) { // :
                mode = MODE_BYTESTRING;
                bytesBuf = Buffer.allocUnsafe(bytesLength);
            }
        } else if (mode === MODE_INT && byte === 101) { // e
            const int = getInt(buf);
            items.push(int);
            mode = MODE_NONE;
            buf = [];
        } else if (mode === MODE_BYTESTRING) {
            bytesBuf[bytesRead++] = byte;
            if (bytesRead === bytesLength) {
                items.push(bytesBuf);
                bytesRead = 0;
                bytesLength = 0;
                mode = MODE_NONE;
            }
        } else {
            buf.push(byte);
        }
        ++i;
    }

    return [items, i];
}

module.exports = function decode(data) {
    return _decode(data)[0];
};