feat: replace instanceOf with unique Symbol checks

src/error/GraphQLError.ts

@@ -29,13 +29,22 @@ export interface GraphQLErrorOptions {
   extensions?: Maybe<GraphQLErrorExtensions>;
 }
 
+const isGraphQLErrorSymbol = Symbol.for('GraphQLError');
+
+export function isGraphQLError(error: unknown): error is GraphQLError {
+  return (
+    typeof error === 'object' && error != null && isGraphQLErrorSymbol in error
+  );
+}
+
 /**
  * A GraphQLError describes an Error found during the parse, validate, or
  * execute phases of performing a GraphQL operation. In addition to a message
  * and stack trace, it also includes information about the locations in a
  * GraphQL document and/or execution result that correspond to the Error.
  */
 export class GraphQLError extends Error {
+  [isGraphQLErrorSymbol]: true = true;
   /**
    * An array of `{ line, column }` locations within the source GraphQL document
    * which correspond to this error.

src/language/source.ts

@@ -1,11 +1,12 @@
 import { devAssert } from '../jsutils/devAssert';
-import { instanceOf } from '../jsutils/instanceOf';
 
 interface Location {
   line: number;
   column: number;
 }
 
+const isSourceSymbol = Symbol.for('Source');
+
 /**
  * A representation of source input to GraphQL. The `name` and `locationOffset` parameters are
  * optional, but they are useful for clients who store GraphQL documents in source files.
@@ -14,6 +15,7 @@ interface Location {
  * The `line` and `column` properties in `locationOffset` are 1-indexed.
  */
 export class Source {
+  [isSourceSymbol]: true = true;
   body: string;
   name: string;
   locationOffset: Location;
@@ -47,5 +49,7 @@ export class Source {
  * @internal
  */
 export function isSource(source: unknown): source is Source {
-  return instanceOf(source, Source);
+  return (
+    typeof source === 'object' && source != null && isSourceSymbol in source
+  );
 }

src/type/__tests__/definition-test.ts

@@ -643,6 +643,27 @@ describe('Type System: Non-Null', () => {
     expectNonNull(ListOfScalarsType).to.not.throw();
     expectNonNull(ListOfNonNullScalarsType).to.not.throw();
   });
+
+  it('rejects a non-type as nullable type of non-null', () => {
+    // @ts-expect-error
+    expectNonNull(NonNullScalarType).to.throw(
+      'Expected Scalar! to be a GraphQL nullable type.',
+    );
+    // @ts-expect-error
+    expectNonNull({}).to.throw('Expected {} to be a GraphQL nullable type.');
+    // @ts-expect-error
+    expectNonNull(String).to.throw(
+      'Expected [function String] to be a GraphQL nullable type.',
+    );
+    // @ts-expect-error (must provide type)
+    expectNonNull(null).to.throw(
+      'Expected null to be a GraphQL nullable type.',
+    );
+    // @ts-expect-error (must provide type)
+    expectNonNull(undefined).to.throw(
+      'Expected undefined to be a GraphQL nullable type.',
+    );
+  });
 });
 
 describe('Type System: test utility methods', () => {