fix: throw upon invalid packet format

Instead of returning an ERROR packet, the decoder will now throw an
error, since it means the other side is sending invalid payloads.

The server will then disconnect the client: [1]

Fixes #3

[1] https://github.com/socketio/socket.io/blob/2.3.0/lib/client.js#L194-L198

Author: darrachequesne

index.js

@@ -13,11 +13,14 @@ exports.ERROR = 4;
 exports.BINARY_EVENT = 5;
 exports.BINARY_ACK = 6;
 
-var errorPacket = {
-  type: exports.ERROR,
-  data: 'parser error'
+var isInteger = Number.isInteger || function (value) {
+  return typeof value === 'number' &&
+    isFinite(value) &&
+    Math.floor(value) === value;
 };
 
+var isString = function (value) { return typeof value === 'string'; };
+
 function Encoder () {}
 
 Encoder.prototype.encode = function (packet, callback) {
@@ -28,13 +31,40 @@ function Decoder () {}
 
 Emitter(Decoder.prototype);
 
+function isDataValid (decoded) {
+  switch (decoded.type) {
+    case exports.CONNECT:
+    case exports.DISCONNECT:
+      return decoded.data === undefined;
+    case exports.ERROR:
+      return isString(decoded.data);
+    default:
+      return Array.isArray(decoded.data);
+  }
+}
+
 Decoder.prototype.add = function (obj) {
-  try {
-    var decoded = JSON.parse(obj);
-    this.emit('decoded', decoded);
-  } catch (e) {
-    this.emit('decoded', errorPacket);
+  var decoded = JSON.parse(obj);
+
+  var isTypeValid = isInteger(decoded.type) && decoded.type >= exports.CONNECT && decoded.type <= exports.BINARY_ACK;
+  if (!isTypeValid) {
+    throw new Error('invalid packet type');
+  }
+
+  if (!isString(decoded.nsp)) {
+    throw new Error('invalid namespace');
   }
+
+  if (!isDataValid(decoded)) {
+    throw new Error('invalid payload');
+  }
+
+  var isAckValid = decoded.id === undefined || isInteger(decoded.id);
+  if (!isAckValid) {
+    throw new Error('invalid packet id');
+  }
+
+  this.emit('decoded', decoded);
 };
 
 Decoder.prototype.destroy = function () {};

test/index.js

@@ -112,4 +112,23 @@ describe('parser', () => {
       done();
     });
   });
+
+  it('throws an error upon invalid format', () => {
+    const decoder = new customParser.Decoder();
+
+    expect(() => decoder.add('{')).to.throwError(/Unexpected end of JSON input/);
+
+    expect(() => decoder.add('{}')).to.throwError(/invalid packet type/);
+    expect(() => decoder.add('{"type":"a"}')).to.throwError(/invalid packet type/);
+    expect(() => decoder.add('{"type":7}')).to.throwError(/invalid packet type/);
+
+    expect(() => decoder.add('{"type":2}')).to.throwError(/invalid namespace/);
+    expect(() => decoder.add('{"type":2,"nsp":2}')).to.throwError(/invalid namespace/);
+
+    expect(() => decoder.add('{"type":2,"nsp":"/"}')).to.throwError(/invalid payload/);
+    expect(() => decoder.add('{"type":2,"nsp":"/","data":4}')).to.throwError(/invalid payload/);
+    expect(() => decoder.add('{"type":4,"nsp":"/","data":[]}')).to.throwError(/invalid payload/);
+
+    expect(() => decoder.add('{"type":2,"nsp":"/","data":[],"id":"a"}')).to.throwError(/invalid packet id/);
+  });
 });