GraphQLError: keep extensions always present

Backport #3313

src/error/GraphQLError.d.ts

@@ -81,7 +81,7 @@ export class GraphQLError extends Error {
   /**
    * Extension fields to add to the formatted error.
    */
-  readonly extensions: { [key: string]: any } | undefined;
+  readonly extensions: { [key: string]: any };
 }
 
 /**

src/error/GraphQLError.js

@@ -61,7 +61,7 @@ export class GraphQLError extends Error {
   /**
    * Extension fields to add to the formatted error.
    */
-  +extensions: { [key: string]: mixed, ... } | void;
+  +extensions: { [key: string]: mixed, ... };
 
   constructor(
     message: string,
@@ -101,7 +101,7 @@ export class GraphQLError extends Error {
         ? positions.map((pos) => getLocation(source, pos))
         : nodeLocations?.map((loc) => getLocation(loc.source, loc.start));
 
-    this.extensions = extensions ?? undefined;
+    this.extensions = extensions ?? {};
 
     const originalExtensions = originalError?.extensions;
     if (isObjectLike(originalExtensions)) {
@@ -119,7 +119,8 @@ export class GraphQLError extends Error {
         enumerable: this.path != null,
       },
       extensions: {
-        enumerable: this.extensions != null,
+        enumerable:
+          this.extensions != null && Object.keys(this.extensions).length > 0,
       },
       name: { enumerable: false },
       nodes: { enumerable: false },

src/error/__tests__/GraphQLError-test.js

@@ -28,10 +28,14 @@ describe('GraphQLError', () => {
     expect(new GraphQLError('str')).to.be.instanceof(GraphQLError);
   });
 
-  it('has a name, message, and stack trace', () => {
+  it('has a name, message, extensions, and stack trace', () => {
     const e = new GraphQLError('msg');
 
-    expect(e).to.include({ name: 'GraphQLError', message: 'msg' });
+    expect(e).to.deep.include({
+      name: 'GraphQLError',
+      message: 'msg',
+      extensions: {},
+    });
     expect(e.stack).to.be.a('string');
   });
 

src/error/formatError.js

@@ -15,7 +15,7 @@ export function formatError(error: GraphQLError): GraphQLFormattedError {
   const path = error.path;
   const extensions = error.extensions;
 
-  return extensions
+  return extensions && Object.keys(extensions).length > 0
     ? { message, locations, path, extensions }
     : { message, locations, path };
 }