Add warning if multiple versions are running (addresses #1470)

packages/core/src/utils.js

@@ -1,2 +1,19 @@
 // @flow
 export let isBrowser = typeof document !== 'undefined'
+
+const globalKey = '__EMOTION_CORE__'
+const globalContext = isBrowser ? window : global
+
+export function checkForMultipleVersions() {
+  if (process.env.NODE_ENV !== 'production') {
+    if (globalContext[globalKey]) {
+      console.warn(
+        'You are loading @emotion/core when it is already loaded. Running' +
+          'multiple instances may cause problems. This can happen if multiple ' +
+          'versions are used, or if multiple builds of the same version are ' +
+          'used.'
+      )
+    }
+    globalContext[globalKey] = true
+  }
+}

packages/utils/src/index.js

@@ -1,7 +1,7 @@
 // @flow
 import type { RegisteredCache, EmotionCache, SerializedStyles } from './types'
 
-let isBrowser = typeof document !== 'undefined'
+const isBrowser = typeof document !== 'undefined'
 
 export function getRegisteredStyles(
   registered: RegisteredCache,