instanceOf: disable dev instanceOf checks if NODE_ENV not set

development behavior should be opt in, not opt out

graphql#4075 (comment)

package.json

@@ -37,7 +37,7 @@
     "test": "npm run lint && npm run check && npm run testonly:cover && npm run prettier:check && npm run check:spelling && npm run check:integrations",
     "lint": "eslint --cache --max-warnings 0 --rulesdir resources/eslint-internal-rules/ .",
     "check": "tsc --pretty",
-    "testonly": "mocha --full-trace src/**/__tests__/**/*-test.ts",
+    "testonly": "dotenvx run --quiet --env NODE_ENV=development -- mocha --full-trace src/**/__tests__/**/*-test.ts",
     "testonly:cover": "c8 npm run testonly",
     "testonly:watch": "npm run testonly -- --watch",
     "prettier": "prettier --cache --cache-strategy metadata --write --list-different .",
@@ -56,6 +56,7 @@
   "devDependencies": {
     "@docusaurus/core": "3.5.2",
     "@docusaurus/preset-classic": "3.5.2",
+    "@dotenvx/dotenvx": "^1.14.0",
     "@mdx-js/react": "3.0.1",
     "@svgr/webpack": "8.1.0",
     "@types/chai": "4.3.19",

src/jsutils/instanceOf.ts

@@ -1,10 +1,10 @@
 import { inspect } from './inspect.js';
 
 /* c8 ignore next 3 */
-const isProduction =
+const isDevelopment =
   globalThis.process != null &&
   // eslint-disable-next-line no-undef
-  process.env.NODE_ENV === 'production';
+  process.env.NODE_ENV === 'development';
 
 /**
  * A replacement for instanceof which includes an error warning when multi-realm
@@ -13,13 +13,8 @@ const isProduction =
  * See: https://webpack.js.org/guides/production/
  */
 export const instanceOf: (value: unknown, constructor: Constructor) => boolean =
-  /* c8 ignore next 6 */
-  // FIXME: https://github.com/graphql/graphql-js/issues/2317
-  isProduction
+  isDevelopment
     ? function instanceOf(value: unknown, constructor: Constructor): boolean {
-        return value instanceof constructor;
-      }
-    : function instanceOf(value: unknown, constructor: Constructor): boolean {
         if (value instanceof constructor) {
           return true;
         }
@@ -50,6 +45,11 @@ spurious results.`,
           }
         }
         return false;
+      }
+    : /* c8 ignore next 4 */
+      // FIXME: https://github.com/graphql/graphql-js/issues/2317
+      function instanceOf(value: unknown, constructor: Constructor): boolean {
+        return value instanceof constructor;
       };
 
 interface Constructor extends Function {