net: improve socket.write() error message

Informative error messages are very important for developers and could
possibly save hours of debugging and frustration. This improves the error
message thrown when writing invalid data into a socket, by communicating
what's expected compared to what the developer just tried to write.

PR-URL: #5981
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/net.js

@@ -619,8 +619,10 @@ Socket.prototype.__defineGetter__('localPort', function() {
 
 
 Socket.prototype.write = function(chunk, encoding, cb) {
-  if (typeof chunk !== 'string' && !(chunk instanceof Buffer))
-    throw new TypeError('Invalid data');
+  if (typeof chunk !== 'string' && !(chunk instanceof Buffer)) {
+    throw new TypeError(
+      'Invalid data, chunk must be a string or buffer, not ' + typeof chunk);
+  }
   return stream.Duplex.prototype.write.apply(this, arguments);
 };
 

test/parallel/test-net-socket-write-error.js

@@ -0,0 +1,18 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const net = require('net');
+
+const server = net.createServer().listen(common.PORT, connectToServer);
+
+function connectToServer() {
+  const client = net.createConnection(common.PORT, () => {
+    assert.throws(() => {
+      client.write(1337);
+    }, /Invalid data, chunk must be a string or buffer, not number/);
+
+    client.end();
+  })
+  .on('end', () => server.close());
+}