From dd0d5e3de4bc6a1e54459057dde4533d20861f46 Mon Sep 17 00:00:00 2001
From: Robert Knight <robert.knight@mendeley.com>
Date: Fri, 3 Apr 2015 10:56:33 +0100
Subject: [PATCH] Warn when multiple instances of React are loaded on the same
 page

This causes a variety of hard-to-debug issues.
See #2402 for examples.

Fixes #2402
---
 grunt/tasks/version-check.js |  2 +-
 src/browser/ui/React.js      | 28 +++++++++++++++++++++++++---
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/grunt/tasks/version-check.js b/grunt/tasks/version-check.js
index 08839a47c9976..b874f5a0d9da7 100644
--- a/grunt/tasks/version-check.js
+++ b/grunt/tasks/version-check.js
@@ -6,7 +6,7 @@ var grunt = require('grunt');
 // Check that the version we're exporting is the same one we expect in the
 // package. This is not an ideal way to do this, but makes sure that we keep
 // them in sync.
-var reactVersionExp = /\bReact\.version\s*=\s*['"]([^'"]+)['"];/;
+var reactVersionExp = /\bREACT_VERSION\s*=\s*['"]([^'"]+)['"];/;
 
 module.exports = function() {
   var reactVersion = reactVersionExp.exec(
diff --git a/src/browser/ui/React.js b/src/browser/ui/React.js
index ccead4fee025c..99aafdc20c734 100644
--- a/src/browser/ui/React.js
+++ b/src/browser/ui/React.js
@@ -13,6 +13,30 @@
 
 'use strict';
 
+var REACT_VERSION = '0.14.0-alpha';
+
+var warning = require('warning');
+
+if (__DEV__) {
+  // before performing any initialization of React, check that
+  // only one instance is in use on the current page.
+  //
+  // Using multiple instances cause a variety of problems if elements
+  // from different versions are mixed on the same page.
+  //
+  // See issue #2402
+  var ExecutionEnvironment = require('ExecutionEnvironment');
+  if (ExecutionEnvironment.canUseDOM) {
+      warning(
+        typeof window.__REACT_VERSION__ === 'undefined',
+        'Multiple instances of React have been initialized on the same page. ' +
+        'Currently initializing React v' + REACT_VERSION + ' but another instance of React v' +
+        window.__REACT_VERSION__ + ' was already initialized'
+      );
+      window.__REACT_VERSION__ = REACT_VERSION;
+  }
+}
+
 var ReactChildren = require('ReactChildren');
 var ReactComponent = require('ReactComponent');
 var ReactClass = require('ReactClass');
@@ -33,7 +57,6 @@ var ReactServerRendering = require('ReactServerRendering');
 var assign = require('Object.assign');
 var findDOMNode = require('findDOMNode');
 var onlyChild = require('onlyChild');
-var warning = require('warning');
 
 ReactDefaultInjection.inject();
 
@@ -50,6 +73,7 @@ if (__DEV__) {
 var render = ReactPerf.measure('React', 'render', ReactMount.render);
 
 var React = {
+  version: REACT_VERSION,
   Children: {
     map: ReactChildren.map,
     forEach: ReactChildren.forEach,
@@ -152,6 +176,4 @@ if (__DEV__) {
   }
 }
 
-React.version = '0.14.0-alpha';
-
 module.exports = React;