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

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

const withNativeBlob =
  typeof Blob === "function" ||
  (typeof Blob !== "undefined" &&
    Object.prototype.toString.call(Blob) === "[object BlobConstructor]");
const withNativeArrayBuffer = typeof ArrayBuffer === "function";

// ArrayBuffer.isView method is not defined in IE10
const isView = obj => {
  return typeof ArrayBuffer.isView === "function"
    ? ArrayBuffer.isView(obj)
    : obj && obj.buffer instanceof ArrayBuffer;
};

const encodePacket = ({ type, data }, supportsBinary, callback) => {
  if (withNativeBlob && data instanceof Blob) {
    if (supportsBinary) {
      return callback(data);
    } else {
      return encodeBlobAsBase64(data, callback);
    }
  } else if (
    withNativeArrayBuffer &&
    (data instanceof ArrayBuffer || isView(data))
  ) {
    if (supportsBinary) {
      return callback(data instanceof ArrayBuffer ? data : data.buffer);
    } else {
      return encodeBlobAsBase64(new Blob([data]), callback);
    }
  }
  // plain string
  return callback(PACKET_TYPES[type] + (data || ""));
};

const encodeBlobAsBase64 = (data, callback) => {
  const fileReader = new FileReader();
  fileReader.onload = function() {
    const content = fileReader.result.split(",")[1];
    callback("b" + content);
  };
  return fileReader.readAsDataURL(data);
};

module.exports = encodePacket;