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

const encodePacket = require("./encodePacket");
const decodePacket = require("./decodePacket");

const SEPARATOR = String.fromCharCode(30); // see https://en.wikipedia.org/wiki/Delimiter#ASCII_delimited_text

const encodePayload = (packets, callback) => {
  // some packets may be added to the array while encoding, so the initial length must be saved
  const length = packets.length;
  const encodedPackets = new Array(length);
  let count = 0;

  packets.forEach((packet, i) => {
    // force base64 encoding for binary packets
    encodePacket(packet, false, encodedPacket => {
      encodedPackets[i] = encodedPacket;
      if (++count === length) {
        callback(encodedPackets.join(SEPARATOR));
      }
    });
  });
};

const decodePayload = (encodedPayload, binaryType) => {
  const encodedPackets = encodedPayload.split(SEPARATOR);
  const packets = [];
  for (let i = 0; i < encodedPackets.length; i++) {
    const decodedPacket = decodePacket(encodedPackets[i], binaryType);
    packets.push(decodedPacket);
    if (decodedPacket.type === "error") {
      break;
    }
  }
  return packets;
};

module.exports = {
  protocol: 4,
  encodePacket,
  encodePayload,
  decodePacket,
  decodePayload
};