Adds check to throw error if non-serializable state is used in a move

integration/package.json

@@ -3,6 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "dependencies": {
+    "boardgame.io": "file:boardgame.io-0.43.2.tgz",
     "react": "^16.7.0",
     "react-dom": "^16.7.0",
     "react-scripts": "^2.1.3"

package.json

@@ -156,6 +156,7 @@
     "koa-body": "^4.1.0",
     "koa-router": "^7.2.1",
     "koa-socket-2": "^1.0.17",
+    "lodash.isplainobject": "^4.0.6",
     "nanoid": "^3.1.20",
     "p-queue": "^6.6.2",
     "prop-types": "^15.5.10",

src/plugins/main.ts

@@ -10,6 +10,7 @@ import PluginImmer from './plugin-immer';
 import PluginRandom from './plugin-random';
 import PluginEvents from './plugin-events';
 import PluginLog from './plugin-log';
+import PluginSerializable from './plugin-serializable';
 import type {
   AnyFn,
   PartialGameState,
@@ -29,7 +30,13 @@ interface PluginOpts {
 /**
  * List of plugins that are always added.
  */
-const DEFAULT_PLUGINS = [PluginImmer, PluginRandom, PluginEvents, PluginLog];
+const DEFAULT_PLUGINS = [
+  PluginImmer,
+  PluginRandom,
+  PluginEvents,
+  PluginLog,
+  PluginSerializable,
+];
 
 /**
  * Allow plugins to intercept actions and process them.

src/plugins/plugin-serializable.test.ts

@@ -0,0 +1,39 @@
+import { Client } from '../client/client';
+
+describe('plugin-serializable', () => {
+  let client;
+
+  beforeAll(() => {
+    const game = {
+      moves: {
+        serializable: () => {
+          return { hello: 'world' };
+        },
+
+        nonSerializable: () => {
+          class Foo {
+            a: number;
+            constructor(a: number) {
+              this.a = a;
+            }
+          }
+          return { hello: new Foo(1) };
+        },
+      },
+    };
+
+    client = Client({ game });
+  });
+
+  test('does not throw for serializable move', () => {
+    expect(() => {
+      client.moves.serializable();
+    }).not.toThrow();
+  });
+
+  test('throws for non-serializable move', () => {
+    expect(() => {
+      client.moves.nonSerializable();
+    }).toThrow();
+  });
+});

src/plugins/plugin-serializable.ts

@@ -0,0 +1,55 @@
+import type { Plugin, AnyFn, Ctx } from '../types';
+import isPlainObject from 'lodash.isplainobject';
+
+/**
+ * Check if a value can be serialized (e.g. using `JSON.stringify`).
+ * Adapted from: https://stackoverflow.com/a/30712764/3829557
+ */
+function isSerializable(value: any) {
+  // Primitives are OK.
+  if (
+    value === undefined ||
+    value === null ||
+    typeof value === 'boolean' ||
+    typeof value === 'number' ||
+    typeof value === 'string'
+  ) {
+    return true;
+  }
+
+  // A non-primitive value that is neither a POJO or an array cannot be serialized.
+  if (!isPlainObject(value) && !Array.isArray(value)) {
+    return false;
+  }
+
+  // Recurse entries if the value is an object or array.
+  for (const key in value) {
+    if (!isSerializable(value[key])) return false;
+  }
+
+  return true;
+}
+
+/**
+ * Plugin that checks whether state is serializable, in order to avoid
+ * network serialization bugs.
+ */
+const SerializablePlugin: Plugin = {
+  name: 'plugin-serializable',
+
+  fnWrap: (move: AnyFn) => (G: unknown, ctx: Ctx, ...args: any[]) => {
+    const result = move(G, ctx, ...args);
+    /* istanbul ignore if */
+    if (process.env.NODE_ENV === 'production') {
+      // Do not perform this check in production.
+      return result;
+    }
+    if (!isSerializable(result)) {
+      throw new Error(`Move state is not JSON-serialiazable. See 
+      https://boardgame.io/documentation/#/?id=state for more information.`);
+    }
+    return result;
+  },
+};
+
+export default SerializablePlugin;