Change type of error extensions from anonymous Record to named interf…

…aces (#3315)

integrationTests/ts/extensions-test.ts

@@ -1,3 +1,4 @@
+import { GraphQLError } from 'graphql/error';
 import { GraphQLString, GraphQLObjectType } from 'graphql/type';
 
 interface SomeExtension {
@@ -50,3 +51,12 @@ const sayHiField = queryType.getFields().sayHi;
 checkExtensionTypes(sayHiField.extensions.someFieldExtension);
 
 checkExtensionTypes(sayHiField.args[0].extensions.someArgumentExtension);
+
+declare module 'graphql' {
+  export interface GraphQLErrorExtensions {
+    someErrorExtension?: SomeExtension;
+  }
+}
+
+const error = new GraphQLError('foo');
+checkExtensionTypes(error.extensions.someErrorExtension);

src/error/GraphQLError.ts

@@ -7,6 +7,19 @@ import type { SourceLocation } from '../language/location';
 import { getLocation } from '../language/location';
 import { printLocation, printSourceLocation } from '../language/printLocation';
 
+/**
+ * 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]: unknown;
+}
+
 /**
  * A GraphQLError describes an Error found during the parse, validate, or
  * execute phases of performing a GraphQL operation. In addition to a message
@@ -61,7 +74,7 @@ export class GraphQLError extends Error {
   /**
    * Extension fields to add to the formatted error.
    */
-  readonly extensions: { [key: string]: unknown };
+  readonly extensions: GraphQLErrorExtensions;
 
   constructor(
     message: string,
@@ -70,7 +83,7 @@ export class GraphQLError extends Error {
     positions?: Maybe<ReadonlyArray<number>>,
     path?: Maybe<ReadonlyArray<string | number>>,
     originalError?: Maybe<Error & { readonly extensions?: unknown }>,
-    extensions?: Maybe<{ [key: string]: unknown }>,
+    extensions?: Maybe<GraphQLErrorExtensions>,
   ) {
     super(message);
 

src/error/index.ts

@@ -1,5 +1,8 @@
 export { GraphQLError, printError, formatError } from './GraphQLError';
-export type { GraphQLFormattedError } from './GraphQLError';
+export type {
+  GraphQLFormattedError,
+  GraphQLErrorExtensions,
+} from './GraphQLError';
 
 export { syntaxError } from './syntaxError';
 

src/index.ts

@@ -382,7 +382,10 @@ export {
   formatError,
 } from './error/index';
 
-export type { GraphQLFormattedError } from './error/index';
+export type {
+  GraphQLFormattedError,
+  GraphQLErrorExtensions,
+} from './error/index';
 
 /** Utilities for operating on GraphQL type schema and parsed sources. */
 export {