polish: introduce spy test helper

src/__testUtils__/__tests__/spyOn-test.ts

@@ -0,0 +1,67 @@
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+import { spyOn, spyOnMethod } from '../spyOn.js';
+
+describe('spyOn', () => {
+  it('tracks invocations while preserving original behavior', () => {
+    const spy = spyOn((a: number, b: number) => a + b);
+
+    expect(spy(2, 3)).to.equal(5);
+    expect(spy(4, 5)).to.equal(9);
+    expect(spy.callCount).to.equal(2);
+  });
+
+  it('preserves this binding', () => {
+    const obj = {
+      base: 10,
+      addToBase: spyOn(function addToBase(
+        this: { base: number },
+        value: number,
+      ) {
+        return this.base + value;
+      }),
+    };
+
+    expect(obj.addToBase(5)).to.equal(15);
+    expect(obj.addToBase.callCount).to.equal(1);
+  });
+});
+
+describe('spyOnMethod', () => {
+  it('tracks method invocations while preserving original behavior', () => {
+    const calculator = {
+      add(a: number, b: number) {
+        return a + b;
+      },
+    };
+
+    const spy = spyOnMethod(calculator, 'add');
+
+    expect(calculator.add(2, 3)).to.equal(5);
+    expect(calculator.add(4, 5)).to.equal(9);
+    expect(spy.callCount).to.equal(2);
+  });
+
+  it('preserves method this binding', () => {
+    const accumulator = {
+      base: 10,
+      addToBase(value: number) {
+        return this.base + value;
+      },
+    };
+
+    const spy = spyOnMethod(accumulator, 'addToBase');
+
+    expect(accumulator.addToBase(5)).to.equal(15);
+    expect(spy.callCount).to.equal(1);
+  });
+
+  it('throws when target property is not a function', () => {
+    const obj: { maybeMethod?: (value: string) => string } = {};
+
+    expect(() => spyOnMethod(obj, 'maybeMethod')).to.throw(
+      "Cannot spy on 'maybeMethod' because it is not a function.",
+    );
+  });
+});

src/__testUtils__/spyOn.ts

@@ -0,0 +1,42 @@
+type AnyFn = (...args: Array<any>) => any;
+
+export interface MethodSpy {
+  readonly callCount: number;
+}
+
+export type SpyFn<T extends AnyFn> = T & MethodSpy;
+
+export function spyOn<T extends AnyFn>(fn: T): SpyFn<T> {
+  let callCount = 0;
+
+  const spy = function (this: unknown, ...args: Parameters<T>): ReturnType<T> {
+    callCount += 1;
+    return fn.apply(this, args) as ReturnType<T>;
+  };
+
+  Object.defineProperty(spy, 'callCount', {
+    get() {
+      return callCount;
+    },
+    enumerable: true,
+  });
+
+  return spy as unknown as SpyFn<T>;
+}
+
+export function spyOnMethod<T extends object>(
+  target: T,
+  key: keyof T,
+): MethodSpy {
+  const original = target[key];
+
+  if (typeof original !== 'function') {
+    throw new Error(
+      `Cannot spy on '${String(key)}' because it is not a function.`,
+    );
+  }
+
+  const spy = spyOn(original as unknown as AnyFn);
+  target[key] = spy as T[keyof T];
+  return spy;
+}

src/execution/__tests__/cancellation-test.ts

@@ -5,6 +5,7 @@ import { expectEqualPromisesOrValues } from '../../__testUtils__/expectEqualProm
 import { expectJSON } from '../../__testUtils__/expectJSON.js';
 import { expectPromise } from '../../__testUtils__/expectPromise.js';
 import { resolveOnNextTick } from '../../__testUtils__/resolveOnNextTick.js';
+import { spyOnMethod } from '../../__testUtils__/spyOn.js';
 
 import { isAsyncIterable } from '../../jsutils/isAsyncIterable.js';
 import { isPromise } from '../../jsutils/isPromise.js';
@@ -747,7 +748,6 @@ describe('Execute: Cancellation', () => {
       promiseWithResolvers<{
         value: () => string;
       }>();
-    let lateValueCalls = 0;
     const sideType = new GraphQLObjectType({
       name: 'LateSide',
       fields: {
@@ -794,15 +794,14 @@ describe('Execute: Cancellation', () => {
     await resolveOnNextTick();
     await resolveOnNextTick();
     const result = await resultPromise;
-    resolveSide({
-      value() {
-        lateValueCalls += 1;
-        return 'late value';
-      },
-    });
+    const lateSide = {
+      value: () => 'late value',
+    };
+    const lateValueSpy = spyOnMethod(lateSide, 'value');
+    resolveSide(lateSide);
     await resolveOnNextTick();
     await resolveOnNextTick();
-    expect(lateValueCalls).to.equal(0);
+    expect(lateValueSpy.callCount).to.equal(0);
 
     expectJSON(result).toDeepEqual({
       data: {
@@ -1067,7 +1066,6 @@ describe('Execute: Cancellation', () => {
     const { promise: nextStarted, resolve: resolveNextStarted } =
       // eslint-disable-next-line @typescript-eslint/no-invalid-void-type
       promiseWithResolvers<void>();
-    let returnCalled = false;
     const asyncIterator = {
       [Symbol.asyncIterator]() {
         return this;
@@ -1077,10 +1075,10 @@ describe('Execute: Cancellation', () => {
         return nextReturned;
       },
       return() {
-        returnCalled = true;
         throw new Error('Return failed');
       },
     };
+    const returnSpy = spyOnMethod(asyncIterator, 'return');
 
     const resultPromise = execute({
       schema,
@@ -1099,7 +1097,7 @@ describe('Execute: Cancellation', () => {
     await expectPromise(resultPromise).toRejectWith(
       'This operation was aborted',
     );
-    expect(returnCalled).to.equal(true);
+    expect(returnSpy.callCount).to.equal(1);
   });
 
   it('ignores async iterator return promise rejections after aborting list completion', async () => {
@@ -1116,7 +1114,6 @@ describe('Execute: Cancellation', () => {
     const { promise: nextStarted, resolve: resolveNextStarted } =
       // eslint-disable-next-line @typescript-eslint/no-invalid-void-type
       promiseWithResolvers<void>();
-    let returnCalled = false;
     const asyncIterator = {
       [Symbol.asyncIterator]() {
         return this;
@@ -1126,10 +1123,10 @@ describe('Execute: Cancellation', () => {
         return nextReturned;
       },
       return() {
-        returnCalled = true;
         return Promise.reject(new Error('Return failed'));
       },
     };
+    const returnSpy = spyOnMethod(asyncIterator, 'return');
 
     const resultPromise = execute({
       schema,
@@ -1148,6 +1145,6 @@ describe('Execute: Cancellation', () => {
     await expectPromise(resultPromise).toRejectWith(
       'This operation was aborted',
     );
-    expect(returnCalled).to.equal(true);
+    expect(returnSpy.callCount).to.equal(1);
   });
 });

src/execution/__tests__/hooks-test.ts

@@ -3,6 +3,7 @@ import { describe, it } from 'mocha';
 
 import { expectPromise } from '../../__testUtils__/expectPromise.js';
 import { resolveOnNextTick } from '../../__testUtils__/resolveOnNextTick.js';
+import { spyOn } from '../../__testUtils__/spyOn.js';
 
 import { isPromise } from '../../jsutils/isPromise.js';
 import type { PromiseOrValue } from '../../jsutils/PromiseOrValue.js';
@@ -470,7 +471,9 @@ describe('Execute: Hooks', () => {
       promiseWithResolvers<ReadonlyArray<string>>();
     const { promise: asyncWorkFinished, resolve: resolveAsyncWorkFinished } =
       promiseWithResolvers<undefined>();
-    let hookCalls = 0;
+    const asyncWorkFinishedSpy = spyOn(() =>
+      resolveAsyncWorkFinished(undefined),
+    );
 
     const result = await experimentalExecuteIncrementally({
       schema: cancellationHookSchema,
@@ -486,10 +489,7 @@ describe('Execute: Hooks', () => {
       `),
       enableEarlyExecution: true,
       hooks: {
-        asyncWorkFinished() {
-          hookCalls += 1;
-          resolveAsyncWorkFinished(undefined);
-        },
+        asyncWorkFinished: asyncWorkFinishedSpy,
       },
       rootValue: {
         todo: {
@@ -532,6 +532,6 @@ describe('Execute: Hooks', () => {
     expect(nextResult.value.hasNext).to.equal(false);
     await asyncWorkFinished;
     await asyncWorkObserved;
-    expect(hookCalls).to.equal(1);
+    expect(asyncWorkFinishedSpy.callCount).to.equal(1);
   });
 });