feat(testing/mock): enable to spy constructors

testing/mock.ts

@@ -555,6 +555,58 @@
   return spy;
 }
 
+/** A constructor wrapper that records all calls made to it. */
+export interface ConstructorSpy<
+  // deno-lint-ignore no-explicit-any
+  Self = any,
+  // deno-lint-ignore no-explicit-any
+  Args extends unknown[] = any[],
+> {
+  new (...args: Args): Self;
+  /** The function that is being spied on. */
+  original: new (...args: Args) => Self;
+  /** Information about calls made to the function or instance method. */
+  calls: SpyCall<Self, Args, Self>[];
+  /** Whether or not the original instance method has been restored. */
+  restored: boolean;
+  /** If spying on an instance method, this restores the original instance method. */
+  restore(): void;
+}
+
+/** Wraps a constructor with a Spy. */
+function constructorSpy<
+  Self,
+  Args extends unknown[],
+>(
+  constructor: new (...args: Args) => Self,
+): ConstructorSpy<Self, Args> {
+  const original = constructor,
+    calls: SpyCall<Self, Args, Self>[] = [];
+  // @ts-ignore TS2509: Can't know the type of `original` statically.
+  const spy = class extends original {
+    constructor(...args: Args) {
+      super(...args);
+      const call: SpyCall<Self, Args, Self> = { args };
+      try {
+        call.returned = this as unknown as Self;
+      } catch (error) {
+        call.error = error as Error;
+        calls.push(call);
+        throw error;
+      }
+      calls.push(call);
+    }
+    static readonly name = original.name;
+    static readonly original = original;
+    static readonly calls = calls;
+    static readonly restored = false;
+    static restore() {
+      throw new MockError("constructor cannot be restored");
+    }
+  } as ConstructorSpy<Self, Args>;
+  return spy;
+}
+
 /** Utility for extracting the arguments type from a property */
 type GetParametersFromProp<
   Self,
@@ -570,6 +622,15 @@
  = Self[Prop] extends (...args: any[]) => infer Return ? Return
   : unknown;
 
+type SpyLike<
+  // deno-lint-ignore no-explicit-any
+  Self = any,
+  // deno-lint-ignore no-explicit-any
+  Args extends unknown[] = any[],
+  // deno-lint-ignore no-explicit-any
+  Return = any,
+> = Spy<Self, Args, Return> | ConstructorSpy<Self, Args>;
+
 /** Wraps a function or instance method with a Spy. */
 export function spy<
   // deno-lint-ignore no-explicit-any
@@ -583,6 +644,13 @@
   Args extends unknown[],
   Return,
 >(func: (this: Self, ...args: Args) => Return): Spy<Self, Args, Return>;
+export function spy<
+  Self,
+  Args extends unknown[],
+  Return = undefined,
+>(
+  constructor: new (...args: Args) => Self,
+): ConstructorSpy<Self, Args>;
 export function spy<
   Self,
   Prop extends keyof Self,
@@ -595,17 +663,23 @@
   Args extends unknown[],
   Return,
 >(
-  funcOrSelf?: ((this: Self, ...args: Args) => Return) | Self,
+  funcOrConstOrSelf?:
+    | ((this: Self, ...args: Args) => Return)
+    | (new (...args: Args) => Self)
+    | Self,
   property?: keyof Self,
-): Spy<Self, Args, Return> {
-  const spy = typeof property !== "undefined"
-    ? methodSpy<Self, Args, Return>(funcOrSelf as Self, property)
-    : typeof funcOrSelf === "function"
-    ? functionSpy<Self, Args, Return>(
-      funcOrSelf as (this: Self, ...args: Args) => Return,
+): SpyLike<Self, Args, Return> {
+  return !funcOrConstOrSelf
+    ? functionSpy<Self, Args, Return>()
+    : property !== undefined
+    ? methodSpy<Self, Args, Return>(funcOrConstOrSelf as Self, property)
+    : funcOrConstOrSelf.toString().startsWith("class")
+    ? constructorSpy<Self, Args>(
+      funcOrConstOrSelf as new (...args: Args) => Self,
     )
-    : functionSpy<Self, Args, Return>();
-  return spy;
+    : functionSpy<Self, Args, Return>(
+      funcOrConstOrSelf as (this: Self, ...args: Args) => Return,
+    );
 }
 
 /** An instance method replacement that records all calls made to it. */
@@ -735,7 +809,7 @@
   Args extends unknown[],
   Return,
 >(
-  spy: Spy<Self, Args, Return>,
+  spy: SpyLike<Self, Args, Return>,
   expectedCalls: number,
 ) {
   try {
@@ -785,7 +859,7 @@
   Args extends unknown[],
   Return,
 >(
-  spy: Spy<Self, Args, Return>,
+  spy: SpyLike<Self, Args, Return>,
   callIndex: number,
   expected?: ExpectedSpyCall<Self, Args, Return>,
 ) {
@@ -864,7 +938,7 @@
   Args extends unknown[],
   Return,
 >(
-  spy: Spy<Self, Args, Promise<Return>>,
+  spy: SpyLike<Self, Args, Promise<Return>>,
   callIndex: number,
   expected?: ExpectedSpyCall<Self, Args, Promise<Return> | Return>,
 ) {
@@ -945,7 +1019,7 @@
   Return,
   ExpectedArg,
 >(
-  spy: Spy<Self, Args, Return>,
+  spy: SpyLike<Self, Args, Return>,
   callIndex: number,
   argIndex: number,
   expected: ExpectedArg,
@@ -969,7 +1043,7 @@
   Return,
   ExpectedArgs extends unknown[],
 >(
-  spy: Spy<Self, Args, Return>,
+  spy: SpyLike<Self, Args, Return>,
   callIndex: number,
   expected: ExpectedArgs,
 ): ExpectedArgs;
@@ -979,7 +1053,7 @@
   Return,
   ExpectedArgs extends unknown[],
 >(
-  spy: Spy<Self, Args, Return>,
+  spy: SpyLike<Self, Args, Return>,
   callIndex: number,
   argsStart: number,
   expected: ExpectedArgs,
@@ -990,7 +1064,7 @@
   Return,
   ExpectedArgs extends unknown[],
 >(
-  spy: Spy<Self, Args, Return>,
+  spy: SpyLike<Self, Args, Return>,
   callIndex: number,
   argStart: number,
   argEnd: number,
@@ -1002,7 +1076,7 @@
   Return,
   Self,
 >(
-  spy: Spy<Self, Args, Return>,
+  spy: SpyLike<Self, Args, Return>,
   callIndex: number,
   argsStart?: number | ExpectedArgs,
   argsEnd?: number | ExpectedArgs,

testing/mock_test.ts

@@ -345,6 +345,66 @@ Deno.test("spy instance method property descriptor", () => {
   assertEquals(action.restored, true);
 });
 
+Deno.test("spy constructor", () => {
+  const PointSpy = spy(Point);
+  assertSpyCalls(PointSpy, 0);
+
+  const point = new PointSpy(2, 3);
+  assertEquals(point.x, 2);
+  assertEquals(point.y, 3);
+  assertEquals(point.action(), undefined);
+
+  assertSpyCall(PointSpy, 0, {
+    self: undefined,
+    args: [2, 3],
+    returned: point,
+  });
+  assertSpyCallArg(PointSpy, 0, 0, 2);
+  assertSpyCallArgs(PointSpy, 0, 0, 1, [2]);
+  assertSpyCalls(PointSpy, 1);
+
+  new PointSpy(3, 5);
+  assertSpyCall(PointSpy, 1, {
+    self: undefined,
+    args: [3, 5],
+  });
+  assertSpyCalls(PointSpy, 2);
+
+  assertThrows(
+    () => PointSpy.restore(),
+    MockError,
+    "constructor cannot be restored",
+  );
+});
+
+Deno.test("spy constructor of child class", () => {
+  const PointSpy = spy(Point);
+  const PointSpyChild = class extends PointSpy {
+    override action() {
+      return 1;
+    }
+  };
+  const point = new PointSpyChild(2, 3);
+
+  assertEquals(point.x, 2);
+  assertEquals(point.y, 3);
+  assertEquals(point.action(), 1);
+
+  assertSpyCall(PointSpyChild, 0, {
+    self: undefined,
+    args: [2, 3],
+    returned: point,
+  });
+  assertSpyCalls(PointSpyChild, 1);
+
+  assertSpyCall(PointSpy, 0, {
+    self: undefined,
+    args: [2, 3],
+    returned: point,
+  });
+  assertSpyCalls(PointSpy, 1);
+});
+
 Deno.test("stub default", () => {
   const point = new Point(2, 3);
   const func = stub(point, "action");