socket.io/packages/engine.io-parser/lib/encodePacket.ts

import { PACKET_TYPES, Packet, RawData } from "./commons.js";

export const encodePacket = (
  { type, data }: Packet,
  supportsBinary: boolean,
  callback: (encodedPacket: RawData) => void,
) => {
  if (data instanceof ArrayBuffer || ArrayBuffer.isView(data)) {
    return callback(
      supportsBinary ? data : "b" + toBuffer(data, true).toString("base64"),
    );
  }
  // plain string
  return callback(PACKET_TYPES[type] + (data || ""));
};

const toBuffer = (data: BufferSource, forceBufferConversion: boolean) => {
  if (
    Buffer.isBuffer(data) ||
    (data instanceof Uint8Array && !forceBufferConversion)
  ) {
    return data;
  } else if (data instanceof ArrayBuffer) {
    return Buffer.from(data);
  } else {
    return Buffer.from(data.buffer, data.byteOffset, data.byteLength);
  }
};

let TEXT_ENCODER;

export function encodePacketToBinary(
  packet: Packet,
  callback: (encodedPacket: RawData) => void,
) {
  if (packet.data instanceof ArrayBuffer || ArrayBuffer.isView(packet.data)) {
    return callback(toBuffer(packet.data, false));
  }
  encodePacket(packet, true, (encoded) => {
    if (!TEXT_ENCODER) {
      // lazily created for compatibility with Node.js 10
      TEXT_ENCODER = new TextEncoder();
    }
    callback(TEXT_ENCODER.encode(encoded));
  });
}