Add arbitrary property to HeaderMap for type differentiation (#7314)

Related: #7313

This shouldn't have been allowed by the typings in the first place.
Previously, a `Map` was compatible with `HeaderMap` according to
TypeScript, so the error wasn't a typing error (when we wanted it to
be). Adding an arbitrary class property to the `HeaderMap` class gives
us the typing error we'd expect when a `Map` is used in lieu of a
`HeaderMap`.

.changeset/great-kids-visit.md

@@ -0,0 +1,12 @@
+---
+'@apollo/server': patch
+---
+
+Add an `__identity` property to `HeaderMap` class to disallow standard `Map`s (in TypeScript).
+
+This ensures that typechecking occurs on fields which are declared to accept a
+`HeaderMap` (notably, the `httpGraphQLRequest.headers` option to
+`ApolloServer.executeHTTPGraphQLRequest` and the `http.headers` option to
+`ApolloServer.executeOperation`). This might be a breaking change for
+integration authors, but should be easily fixed by switching from `new
+Map<string, string>()` to `new HeaderMap()`.

docs/source/integrations/building-integrations.md

@@ -149,7 +149,7 @@ With the request body parsed, we can now construct an `HTTPGraphQLRequest`:
 ```ts
 interface HTTPGraphQLRequest {
   method: string;
-  headers: Map<string, string>;
+  headers: HeaderMap; // the `HeaderMap` class is exported by @apollo/server
   search: string;
   body: unknown;
 }
@@ -164,7 +164,9 @@ Finally, we have to create the `headers` property because Apollo Server expects
 In the Express integration, we construct a `Map` by iterating over the `headers` object, like so:
 
 ```ts
-const headers = new Map<string, string>();
+import { HeaderMap } from '@apollo/server';
+
+const headers = new HeaderMap();
 for (const [key, value] of Object.entries(req.headers)) {
   if (value !== undefined) {
     headers.set(key, Array.isArray(value) ? value.join(', ') : value);
@@ -221,7 +223,7 @@ After awaiting the Promise returned by `executeHTTPGraphQLRequest`, we receive a
 ```ts
 interface HTTPGraphQLHead {
   status?: number;
-  headers: Map<string, string>;
+  headers: HeaderMap;
 }
 
 type HTTPGraphQLResponseBody =

packages/server/src/__tests__/runQuery.test.ts

@@ -837,7 +837,7 @@ describe('request pipeline life-cycle hooks', () => {
               message: 'Syntax Error: Expected Name, found "}".',
               extensions: {
                 code: 'GRAPHQL_PARSE_FAILED',
-                http: { status: 400, headers: new HeaderMap() },
+                http: { status: 400, headers: expect.any(HeaderMap) },
               },
             }),
           ]),
@@ -862,7 +862,7 @@ describe('request pipeline life-cycle hooks', () => {
                 'Cannot query field "testStringWithParseError" on type "QueryType".',
               extensions: {
                 code: 'GRAPHQL_VALIDATION_FAILED',
-                http: { status: 400, headers: new HeaderMap() },
+                http: { status: 400, headers: expect.any(HeaderMap) },
               },
             }),
           ]),

packages/server/src/utils/HeaderMap.ts

@@ -1,4 +1,9 @@
 export class HeaderMap extends Map<string, string> {
+  // In order for TypeScript to prevent a standard `Map` from being compatible
+  // with a `HeaderMap`, we need some additional property on the class.
+  // @ts-ignore (this is just unused)
+  private __identity = Symbol('HeaderMap');
+
   override set(key: string, value: string): this {
     return super.set(key.toLowerCase(), value);
   }