Track policies.rootIdsByTypename as well as policies.rootTypenamesById.

Believe it or not, iterating over the values of policies.rootTypenamesById
was noticeably expensive according to Chrome devtools profiling. Since
this information almost never changes, we might as well maintain it in the
format that's most convenient.

src/cache/inmemory/policies.ts

@@ -212,12 +212,8 @@ export class Policies {
     };
   } = Object.create(null);
 
-  public readonly rootTypenamesById: Readonly<Record<string, string>> = {
-    __proto__: null, // Equivalent to Object.create(null)
-    ROOT_QUERY: "Query",
-    ROOT_MUTATION: "Mutation",
-    ROOT_SUBSCRIPTION: "Subscription",
-  };
+  public readonly rootIdsByTypename: Record<string, string> = Object.create(null);
+  public readonly rootTypenamesById: Record<string, string> = Object.create(null);
 
   public readonly usingPossibleTypes = false;
 
@@ -231,6 +227,10 @@ export class Policies {
       ...config,
     };
 
+    this.setRootTypename("Query");
+    this.setRootTypename("Mutation");
+    this.setRootTypename("Subscription");
+
     if (config.possibleTypes) {
       this.addPossibleTypes(config.possibleTypes);
     }
@@ -346,13 +346,14 @@ export class Policies {
 
   private setRootTypename(
     which: "Query" | "Mutation" | "Subscription",
-    typename: string,
+    typename: string = which,
   ) {
     const rootId = "ROOT_" + which.toUpperCase();
     const old = this.rootTypenamesById[rootId];
     if (typename !== old) {
-      invariant(old === which, `Cannot change root ${which} __typename more than once`);
-      (this.rootTypenamesById as any)[rootId] = typename;
+      invariant(!old || old === which, `Cannot change root ${which} __typename more than once`);
+      this.rootIdsByTypename[typename] = rootId;
+      this.rootTypenamesById[rootId] = typename;
     }
   }
 

src/cache/inmemory/readFromStore.ts

@@ -206,9 +206,7 @@ export class StoreReader {
 
     if (this.config.addTypename &&
         typeof typename === "string" &&
-        Object.values(
-          policies.rootTypenamesById
-        ).indexOf(typename) < 0) {
+        !policies.rootIdsByTypename[typename]) {
       // Ensure we always include a default value for the __typename
       // field, if we have one, and this.config.addTypename is true. Note
       // that this field can be overridden by other merged objects.