A websocket frame parser in javascript

WebSocketFrame.js

class WSFrame {
  constructor(buffer) {

    if(buffer.byteLength < 2) {
      throw new Error('Incomplete frame: Header data missing');
    }

    this.fin = 0;
    this.opcode = 0;
    this.mask = 0;
    this.payloadLength = 0;
    this.maskKey = null;
    this.length = 0;
    this._message = null;
    this.frameLength = 0;

    this.fin = buffer[0] >> 7;
    this.opcode = buffer[0] & 0b1111;
    this.payloadLength = buffer[1] & 0b1111111;
    this.mask = buffer[1] >> 7;

    buffer = buffer.slice(2);

    if(this.payloadLength < 126) {
      this.length = this.payloadLength;

      if(this.mask) {
        this.frameLength = 6 + this.length;
      } else {
        this.frameLength = 2 + this.length;
      }

      if(buffer.byteLength < 4 && this.mask) {
        throw new Error('Incomplete frame: Mask key missing');
      }

      if(this.mask) {
        this.maskKey = buffer.slice(0, 4);
        buffer = buffer.slice(4);
      }
      else {
        buffer = buffer.slice(0, this.length);
      }

    } else if(this.payloadLength == 126) {

      if(buffer.byteLength < 6 && this.mask) {
        throw new Error('Incomplete frame: Mask key missing');
      }

      let bytes = [
        [0, 1],
        [1, 0]
      ];

      for(let b of bytes) {
        let k, i;
        for(k = 0, i = 1; i < b.length ; i++, k++) {
          this.length += buffer[k] * 1 << (8*i);
        }
      }

      if(this.mask) {
        this.frameLength = 8 + this.length;
      }
      else {
        this.frameLength = 4 + this.length;
      }

      buffer = buffer.slice(2);

      if(this.mask) {
        this.maskKey = buffer.slice(0, 4);
        buffer = buffer.slice(4);
      }
      else {
        buffer = buffer.slice(0, this.length);
      }

    } else {

      if(buffer.byteLength < 10 && this.mask) {
        throw new Error('Incomplete frame: Mask key missing');
      }

      let bytes = [
        [0, 7],
        [1, 6],
        [2, 5],
        [3, 4],
        [4, 3],
        [5, 2],
        [6, 1],
        [7, 0]
      ];

      for(let b of bytes) {
        let k, i;
        for(k = 0, i = 1; i < b.length ; i++, k++) {
          this.length += buffer[k] * 1 << (8*i);
        }
      }

      if(this.mask) {
        this.frameLength = 14 + this.length;
      } else {
        this.frameLength = 10 + this.length;
      }

      buffer = buffer.slice(8);

      if(this.mask) {
        this.maskKey = buffer.slice(0, 4);
        buffer = buffer.slice(4);
      } else {
        buffer = buffer[this.length];
      }

    }
    this._message = buffer;

  }

  get message() {
    if(!this.mask) {
      return this._message.toString('utf8');
    } else {
      let message = Buffer.alloc(this._message.length);
      for(let i = 0; i < this.length; i++) {
        message[i] = this._message[i] ^ this.maskKey[i % 4];
      }
      return message.toString('utf8');
    }
  }

}