node_modules/engine.io-parser/lib/encodePacket.js

const { PACKET_TYPES } = require("./commons");

const encodePacket = ({ type, data }, supportsBinary, callback) => {
  if (data instanceof ArrayBuffer || ArrayBuffer.isView(data)) {
    const buffer = toBuffer(data);
    return callback(encodeBuffer(buffer, supportsBinary));
  }
  // plain string
  return callback(PACKET_TYPES[type] + (data || ""));
};

const toBuffer = data => {
  if (Buffer.isBuffer(data)) {
    return data;
  } else if (data instanceof ArrayBuffer) {
    return Buffer.from(data);
  } else {
    return Buffer.from(data.buffer, data.byteOffset, data.byteLength);
  }
};

// only 'message' packets can contain binary, so the type prefix is not needed
const encodeBuffer = (data, supportsBinary) => {
  return supportsBinary ? data : "b" + data.toString("base64");
};

module.exports = encodePacket;