Add failure messages to gin and js mojom encoder

mojo/public/js/codec.js

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 define("mojo/public/js/codec", [
   "mojo/public/js/unicode",
   "mojo/public/js/buffer",
 ], function(unicode, buffer) {
 
   var kErrorUnsigned = "Passing negative value to unsigned";
+  var kErrorArray = "Passing non-array-like to array";
 
   // Memory -------------------------------------------------------------------
 
   var kAlignment = 8;
 
   function align(size) {
     return size + (kAlignment - (size % kAlignment)) % kAlignment;
   }
 
   function isAligned(offset) {
@@ skipping 324 matching lines @@
     var encoder = this.createAndEncodeEncoder(cls.encodedSize);
     cls.encode(encoder, val);
   };
 
   Encoder.prototype.encodeArrayPointer = function(cls, val) {
     if (val == null) {
       // Also handles undefined, since undefined == null.
       this.encodePointer(val);
       return;
     }
+    if (val.length === undefined) {
+      throw new Error(kErrorArray);

[hansmuller, 2014/12/15 16:56:17]

I suppose another indication that val isn't an array (if val.length was defined)
would be that val.length < 0. But shouldn't you be using |val instanceof Array|
here?

+    }
     var numberOfElements = val.length;
     var encodedSize = kArrayHeaderSize + ((cls === PackedBool) ?
         Math.ceil(numberOfElements / 8) : cls.encodedSize * numberOfElements);
     var encoder = this.createAndEncodeEncoder(encodedSize);
     encoder.encodeArray(cls, val, numberOfElements, encodedSize);
   };
 
   Encoder.prototype.encodeStringPointer = function(val) {
     if (val == null) {
       // Also handles undefined, since undefined == null.
@@ skipping 441 matching lines @@
   exports.NullablePointerTo = NullablePointerTo;
   exports.ArrayOf = ArrayOf;
   exports.NullableArrayOf = NullableArrayOf;
   exports.PackedBool = PackedBool;
   exports.Handle = Handle;
   exports.NullableHandle = NullableHandle;
   exports.MapOf = MapOf;
   exports.NullableMapOf = NullableMapOf;
   return exports;
 });