Change type of error extensions from anonymous Record to named interfaces

src/error/GraphQLError.d.ts

@@ -4,6 +4,19 @@ import { ASTNode } from '../language/ast';
 import { Source } from '../language/source';
 import { SourceLocation } from '../language/location';
 
+/**
+ * Custom extensions
+ *
+ * @remarks
+ * Use a unique identifier name for your extension, for example the name of
+ * your library or project. Do not use a shortened identifier as this increases
+ * the risk of conflicts. We recommend you add at most one extension field,
+ * an object which can contain all the values you need.
+ */
+export interface GraphQLErrorExtensions {
+  [attributeName: string]: any;
+}
+
 /**
  * A GraphQLError describes an Error found during the parse, validate, or
  * execute phases of performing a GraphQL operation. In addition to a message
@@ -18,7 +31,7 @@ export class GraphQLError extends Error {
     positions?: Maybe<ReadonlyArray<number>>,
     path?: Maybe<ReadonlyArray<string | number>>,
     originalError?: Maybe<Error>,
-    extensions?: Maybe<{ [key: string]: any }>,
+    extensions?: Maybe<{ [key: string]: GraphQLErrorExtensions }>,
   );
 
   /**

src/error/index.d.ts

@@ -1,4 +1,8 @@
-export { GraphQLError, printError } from './GraphQLError';
+export {
+  GraphQLError,
+  GraphQLErrorExtensions,
+  printError,
+} from './GraphQLError';
 export { syntaxError } from './syntaxError';
 export { locatedError } from './locatedError';
 export { formatError, GraphQLFormattedError } from './formatError';

src/index.d.ts

@@ -364,6 +364,7 @@ export {
   printError,
   formatError,
   GraphQLFormattedError,
+  GraphQLErrorExtensions,
 } from './error/index';
 
 // Utilities for operating on GraphQL type schema and parsed sources.