Throw error when attempting to redefine ReactOnRails global

Webpack configuration could attempt to import react-on-rails multiple times in
each runtime chunk, leading to the global ReactOnRails variable being
overridden and client rendering crashes. This should be avoided by relying on a
single runtime chunk.

Read more at #1558.

docs/javascript/troubleshooting-when-using-webpacker.md

@@ -1,3 +1,83 @@
+# Client rendering crashes when configuring `optimization.runtimeChunk` to `multiple`
+
+## Context
+
+1. Ruby version: 3.1
+2. Rails version: 7.0.6
+3. Shakapacker version: 6.6.0
+4. React on Rails version: 13.3.5
+
+## The failure
+
+Configuring webpack to embed the runtime in each chunk and calling `react_component` twice in a rails view/partial causes the client render to crash with the following error:
+
+```
+Could not find component registered with name XXX. Registered component names include [ YYY ]. Maybe you forgot to register the component?
+```
+
+```
+VM4859 clientStartup.js:132 Uncaught Error: ReactOnRails encountered an error while rendering component: XXX. See above error message.
+    at Object.get (ComponentRegistry.js:40:15)
+    at Object.getComponent (ReactOnRails.js:211:44)
+    at render (VM4859 clientStartup.js:103:53)
+    at forEachReactOnRailsComponentRender (VM4859 clientStartup.js:138:9)
+    at reactOnRailsPageLoaded (VM4859 clientStartup.js:164:5)
+    at renderInit (VM4859 clientStartup.js:205:9)
+    at onPageReady (VM4859 clientStartup.js:234:9)
+    at HTMLDocument.onReadyStateChange (VM4859 clientStartup.js:238:13)
+```
+
+## Configs
+
+### Webpack configuration
+
+```js
+optimization: {
+  runtimeChunk: 'multiple'
+},
+```
+
+### Rails view
+
+```haml
+= react_component("XXX", props: @props)
+= yield
+= react_component("YYY", props: @props)
+```
+
+## The problem
+
+Configuring webpack to embed the runtime in each chunk and calling `react_component` twice in a rails view/partial causes the client render to crash.
+
+Read more at https://github.com/shakacode/react_on_rails/issues/1558.
+
+## Solution
+
+To overcome this issue, we could use [shakapacker](https://github.com/shakacode/shakapacker)'s default optimization configuration (pseudo-code):
+
+```js
+const { webpackConfig: baseClientWebpackConfig } = require('shakapacker');
+
+// ...
+
+config.optimization = baseClientWebpackConfig.optimization;
+```
+As it set the `optimization.runtimeChunk` to `single`. See its source:
+
+`package/environments/base.js:115`
+```js
+  optimization: {
+    splitChunks: { chunks: 'all' },
+
+    runtimeChunk: 'single'
+  },
+```
+https://github.com/shakacode/shakapacker/blob/cdf32835d3e0949952b8b4b53063807f714f9b24/package/environments/base.js#L115-L119
+
+Or set `optimization.runtimeChunk` to `single` directly.
+
+# When `ReactOnRails.configuration.webpack_generated_files` is specified, it prevents usage of `manifest.json`
+
 ## Context
 
 Rails: 5.0.2

node_package/src/ReactOnRails.ts

@@ -30,6 +30,16 @@ if (ctx === undefined) {
   throw new Error("The context (usually Window or NodeJS's Global) is undefined.");
 }
 
+if (ctx.ReactOnRails !== undefined) {
+  throw new Error(`
+    The ReactOnRails value exists in the ${ctx} scope, it may not be safe to overwrite it.
+    
+    This could be caused by setting Webpack's optimization.runtimeChunk to "true" or "multiple" - please check your Webpack configuration.
+    
+    Read more at https://github.com/shakacode/react_on_rails/issues/1558.
+  `);
+}
+
 const DEFAULT_OPTIONS = {
   traceTurbolinks: false,
   turbo: false,