node_modules/engine.io-parser/lib/decodePacket.browser.js

const { PACKET_TYPES_REVERSE, ERROR_PACKET } = require("./commons");

const withNativeArrayBuffer = typeof ArrayBuffer === "function";

let base64decoder;
if (withNativeArrayBuffer) {
  base64decoder = require("base64-arraybuffer");
}

const decodePacket = (encodedPacket, binaryType) => {
  if (typeof encodedPacket !== "string") {
    return {
      type: "message",
      data: mapBinary(encodedPacket, binaryType)
    };
  }
  const type = encodedPacket.charAt(0);
  if (type === "b") {
    return {
      type: "message",
      data: decodeBase64Packet(encodedPacket.substring(1), binaryType)
    };
  }
  const packetType = PACKET_TYPES_REVERSE[type];
  if (!packetType) {
    return ERROR_PACKET;
  }
  return encodedPacket.length > 1
    ? {
        type: PACKET_TYPES_REVERSE[type],
        data: encodedPacket.substring(1)
      }
    : {
        type: PACKET_TYPES_REVERSE[type]
      };
};

const decodeBase64Packet = (data, binaryType) => {
  if (base64decoder) {
    const decoded = base64decoder.decode(data);
    return mapBinary(decoded, binaryType);
  } else {
    return { base64: true, data }; // fallback for old browsers
  }
};

const mapBinary = (data, binaryType) => {
  switch (binaryType) {
    case "blob":
      return data instanceof ArrayBuffer ? new Blob([data]) : data;
    case "arraybuffer":
    default:
      return data; // assuming the data is already an ArrayBuffer
  }
};

module.exports = decodePacket;