Only trigger "one instance of graphql" error in DEV environments.

This ensures the additional checking for multiple graphql instances only occurs in non-production builds. Not only does this improve the performance for production builds, it also solves a spurious error case for minified builds:

Since this additional check compares the `.name` of the constructors as a proxy for determining if two classes are representationally equal, minified builds may result in spurious false-positives if class names are minified to single-character names and those characters are likely to conflict.

This doesn't require `process` to exist (defaulting to the non-production mode), but bundlers like Webpack should consider using a stripping transform (see: https://webpack.js.org/guides/production/#specify-the-environment) in production builds if they also use a minifier.

src/jsutils/instanceOf.js

@@ -16,29 +16,35 @@ declare function instanceOf(
   constructor: mixed,
 ): boolean %checks(value instanceof constructor);
 
-// eslint-disable-next-line no-redeclare
-export default function instanceOf(value, constructor) {
-  if (value instanceof constructor) {
-    return true;
-  }
-  if (value) {
-    const valueConstructor = value.constructor;
-    if (valueConstructor && valueConstructor.name === constructor.name) {
-      throw new Error(
-        `Cannot use ${constructor.name} "${value}" from another module or realm.
+export default (process && process.env.NODE_ENV !== 'production'
+  ? // eslint-disable-next-line no-shadow
+    function instanceOf(value: any, constructor: any) {
+      if (value instanceof constructor) {
+        return true;
+      }
+      if (value) {
+        const valueClass = value.constructor;
+        const className = constructor.name;
+        if (valueClass && valueClass.name === className) {
+          throw new Error(
+            `Cannot use ${className} "${value}" from another module or realm.
 
-Ensure that there is only one instance of "graphql" in the node_modules
-directory. If different versions of "graphql" are the dependencies of other
-relied on modules, use "resolutions" to ensure only one version is installed.
+    Ensure that there is only one instance of "graphql" in the node_modules
+    directory. If different versions of "graphql" are the dependencies of other
+    relied on modules, use "resolutions" to ensure only one version is installed.
 
-https://yarnpkg.com/en/docs/selective-version-resolutions
+    https://yarnpkg.com/en/docs/selective-version-resolutions
 
-Duplicate "graphql" modules cannot be used at the same time since different
-versions may have different capabilities and behavior. The data from one
-version used in the function from another could produce confusing and
-spurious results.`,
-      );
+    Duplicate "graphql" modules cannot be used at the same time since different
+    versions may have different capabilities and behavior. The data from one
+    version used in the function from another could produce confusing and
+    spurious results.`,
+          );
+        }
+      }
+      return false;
     }
-  }
-  return false;
-}
+  : // eslint-disable-next-line no-shadow
+    function instanceOf(value: any, constructor: any) {
+      return value instanceof constructor;
+    });