WebSockets server tutorial

server.js

function constructReply (data) {
  // Convert the data to JSON and copy it into a buffer
  const json = JSON.stringify(data)
  const jsonByteLength = Buffer.byteLength(json);
  // Note: we're not supporting > 65535 byte payloads at this stage 
  const lengthByteCount = jsonByteLength < 126 ? 0 : 2; 
  const payloadLength = lengthByteCount === 0 ? jsonByteLength : 126; 
  const buffer = Buffer.alloc(2 + lengthByteCount + jsonByteLength); 
  // Write out the first byte, using opcode `1` to indicate that the message 
  // payload contains text data 
  buffer.writeUInt8(0b10000001, 0); 
  buffer.writeUInt8(payloadLength, 1); 
  // Write the length of the JSON payload to the second byte 
  let payloadOffset = 2; 
  if (lengthByteCount > 0) { 
    buffer.writeUInt16BE(jsonByteLength, 2); payloadOffset += lengthByteCount; 
  } 
  // Write the JSON data to the data buffer 
  buffer.write(json, payloadOffset); 
  return buffer;
}