Enable strict mode in tsconfig and fix type errors

src/cache/inmemory/__tests__/writeToStore.ts

@@ -26,8 +26,13 @@ import { InMemoryCache } from "../inMemoryCache";
 import { withErrorSpy, withWarningSpy } from "../../../testing";
 import { TypedDocumentNode } from "../../../core";
 import { extractFragmentContext } from "../helpers";
+import { KeyFieldsFunction } from "../policies";
+import { invariant } from "../../../utilities/globals";
 
-const getIdField = ({ id }: { id: string }) => id;
+const getIdField: KeyFieldsFunction = ({ id }) => {
+  invariant(typeof id === "string", "id is not a string");
+  return id;
+};
 
 describe("writing to the store", () => {
   const cache = new InMemoryCache({
@@ -1293,19 +1298,17 @@ describe("writing to the store", () => {
     }
 
     testData.forEach((data) => {
-      data.mutation.definitions.forEach(
-        (definition: OperationDefinitionNode) => {
-          if (isOperationDefinition(definition)) {
-            definition.selectionSet.selections.forEach((selection) => {
-              if (isField(selection)) {
-                expect(
-                  storeKeyNameFromField(selection, data.variables)
-                ).toEqual(data.expected);
-              }
-            });
-          }
+      data.mutation.definitions.forEach((definition) => {
+        if (isOperationDefinition(definition)) {
+          definition.selectionSet.selections.forEach((selection) => {
+            if (isField(selection)) {
+              expect(storeKeyNameFromField(selection, data.variables)).toEqual(
+                data.expected
+              );
+            }
+          });
         }
-      );
+      });
     });
   });
 

src/cache/inmemory/readFromStore.ts

@@ -535,7 +535,7 @@ function firstMissing(tree: MissingTree): string | undefined {
       return value;
     });
   } catch (result) {
-    return result;
+    return result as string;
   }
 }
 

src/link/error/index.ts

@@ -84,7 +84,7 @@ export function onError(errorHandler: ErrorHandler): ApolloLink {
           },
         });
       } catch (e) {
-        errorHandler({ networkError: e, operation, forward });
+        errorHandler({ networkError: e as Error, operation, forward });
         observer.error(e);
       }
 

src/react/hooks/useQuery.ts

@@ -271,8 +271,8 @@ class InternalState<TData, TVariables extends OperationVariables> {
   // useQuery method, so we can safely use these members in other/later methods
   // without worrying they might be uninitialized.
   private renderPromises: ApolloContextValue["renderPromises"];
-  private queryHookOptions: QueryHookOptions<TData, TVariables>;
-  private watchQueryOptions: WatchQueryOptions<TVariables, TData>;
+  private queryHookOptions!: QueryHookOptions<TData, TVariables>;
+  private watchQueryOptions!: WatchQueryOptions<TVariables, TData>;
 
   private useOptions(options: QueryHookOptions<TData, TVariables>) {
     const watchQueryOptions = this.createWatchQueryOptions(
@@ -461,8 +461,8 @@ class InternalState<TData, TVariables extends OperationVariables> {
   private onCompleted(data: TData) {}
   private onError(error: ApolloError) {}
 
-  private observable: ObservableQuery<TData, TVariables>;
-  private obsQueryFields: Omit<
+  private observable!: ObservableQuery<TData, TVariables>;
+  private obsQueryFields!: Omit<
     ObservableQueryFields<TData, TVariables>,
     "variables"
   >;

src/testing/core/itAsync.ts

@@ -9,7 +9,7 @@ function wrap(key?: "only" | "skip" | "todo") {
   ) =>
     (key ? it[key] : it)(
       message,
-      function () {
+      function (this: unknown) {
         return new Promise((resolve, reject) =>
           callback.call(this, resolve, reject)
         );
@@ -21,7 +21,7 @@ function wrap(key?: "only" | "skip" | "todo") {
 const wrappedIt = wrap();
 
 export const itAsync = Object.assign(
-  function (...args: Parameters<typeof wrappedIt>) {
+  function (this: unknown, ...args: Parameters<typeof wrappedIt>) {
     return wrappedIt.apply(this, args);
   },
   {

src/testing/core/mocking/mockLink.ts

@@ -45,7 +45,7 @@ function requestToKey(request: GraphQLRequest, addTypename: Boolean): string {
 }
 
 export class MockLink extends ApolloLink {
-  public operation: Operation;
+  public operation!: Operation;
   public addTypename: Boolean = true;
   public showWarnings: boolean = true;
   private mockedResponsesByKey: { [key: string]: MockedResponse[] } = {};

src/utilities/graphql/transform.ts

@@ -12,7 +12,7 @@ import type {
   FragmentSpreadNode,
   VariableDefinitionNode,
   ASTNode,
-  ASTVisitor,
+  ASTVisitFn,
   InlineFragmentNode,
 } from "graphql";
 import { visit, Kind } from "graphql";
@@ -29,6 +29,12 @@ import type { FragmentMap } from "./fragments.js";
 import { createFragmentMap } from "./fragments.js";
 import { isArray, isNonEmptyArray } from "../common/arrays.js";
 
+// https://github.com/graphql/graphql-js/blob/8d7c8fccf5a9846a50785de04abda58a7eb13fc0/src/language/visitor.ts#L20-L23
+interface EnterLeaveVisitor<TVisitedNode extends ASTNode> {
+  readonly enter?: ASTVisitFn<TVisitedNode>;
+  readonly leave?: ASTVisitFn<TVisitedNode>;
+}
+
 export type RemoveNodeConfig<N> = {
   name?: string;
   test?: (node: N) => boolean;
@@ -208,8 +214,10 @@ export function removeDirectivesFromDocument(
   // original doc immediately without any modifications.
   let firstVisitMadeChanges = false;
 
-  const fieldOrInlineFragmentVisitor: ASTVisitor = {
-    enter(node: FieldNode | InlineFragmentNode) {
+  const fieldOrInlineFragmentVisitor: EnterLeaveVisitor<
+    FieldNode | InlineFragmentNode
+  > = {
+    enter(node) {
       if (shouldRemoveField(node.directives)) {
         firstVisitMadeChanges = true;
         return null;
@@ -385,8 +393,10 @@ export function removeDirectivesFromDocument(
       )
     );
 
-  const enterVisitor: ASTVisitor = {
-    enter(node: FragmentSpreadNode | FragmentDefinitionNode) {
+  const enterVisitor: EnterLeaveVisitor<
+    FragmentSpreadNode | FragmentDefinitionNode
+  > = {
+    enter(node) {
       if (fragmentWillBeRemoved(node.name.value)) {
         return null;
       }

src/utilities/observables/Concast.ts

@@ -147,9 +147,9 @@ export class Concast<T> extends Observable<T> {
   // Any Concast object can be trivially converted to a Promise, without
   // having to create a new wrapper Observable. This promise provides an
   // easy way to observe the final state of the Concast.
-  private resolve: (result?: T | PromiseLike<T>) => void;
-  private reject: (reason: any) => void;
-  public readonly promise = new Promise<T>((resolve, reject) => {
+  private resolve!: (result?: T | PromiseLike<T>) => void;
+  private reject!: (reason: any) => void;
+  public readonly promise = new Promise<T | undefined>((resolve, reject) => {
     this.resolve = resolve;
     this.reject = reject;
   });

src/utilities/policies/pagination.ts

@@ -42,7 +42,7 @@ export function offsetLimitPagination<T = Reference>(
           // to receive any arguments, so you might prefer to throw an
           // exception here, instead of recovering by appending incoming
           // onto the existing array.
-          merged.push.apply(merged, incoming);
+          merged.push(...incoming);
         }
       }
 

tsconfig.json

@@ -17,7 +17,8 @@
     "experimentalDecorators": true,
     "outDir": "./dist",
     "lib": ["es2015", "esnext.asynciterable", "dom"],
-    "jsx": "react"
+    "jsx": "react",
+    "strict": true
   },
   "include": ["src/**/*.ts", "src/**/*.tsx"],
   "exclude": ["src/**/__tests__/**/*"]