From d88856c3fa0ebadef490a46b44caeacf7eec8e4b Mon Sep 17 00:00:00 2001
From: Lee Donghyun <d@donghyun.cc>
Date: Mon, 26 Jan 2026 00:38:25 +0900
Subject: [PATCH] test(common): add unit tests for class serializer interceptor

Add comprehensive unit tests for ClassSerializerInterceptor which
previously had no test coverage despite being a public API.

Test coverage includes:
- constructor with custom transformer package injection
- intercept() with RxJS pipe and context options merging
- serialize() for arrays, objects, and StreamableFile handling
- transformToPlain() with type conversion and instanceOf checks
- getContextOptions() with metadata precedence (handler > class)
- Edge cases: null/undefined, mixed arrays, Date objects, primitives

Total: 25+ test cases covering all public methods and edge cases.
---
 .../class-serializer.interceptor.spec.ts      | 468 ++++++++++++++++++
 1 file changed, 468 insertions(+)
 create mode 100644 packages/common/test/serializer/class-serializer.interceptor.spec.ts

diff --git a/packages/common/test/serializer/class-serializer.interceptor.spec.ts b/packages/common/test/serializer/class-serializer.interceptor.spec.ts
new file mode 100644
index 000000000..5ebc74fbc
--- /dev/null
+++ b/packages/common/test/serializer/class-serializer.interceptor.spec.ts
@@ -0,0 +1,468 @@
+import { expect } from 'chai';
+import * as sinon from 'sinon';
+import { of, throwError } from 'rxjs';
+import { ClassSerializerInterceptor } from '../../serializer/class-serializer.interceptor';
+import { ExecutionContext, CallHandler } from '../../interfaces';
+import { StreamableFile } from '../../file-stream';
+
+describe('ClassSerializerInterceptor', () => {
+  let interceptor: ClassSerializerInterceptor;
+  let mockReflector: any;
+  let mockTransformerPackage: any;
+  let sandbox: sinon.SinonSandbox;
+
+  beforeEach(() => {
+    sandbox = sinon.createSandbox();
+    mockReflector = {
+      getAllAndOverride: sandbox.stub(),
+    };
+    mockTransformerPackage = {
+      classToPlain: sandbox.stub(),
+      plainToInstance: sandbox.stub(),
+    };
+  });
+
+  afterEach(() => {
+    sandbox.restore();
+  });
+
+  describe('constructor', () => {
+    it('should create interceptor with default transformer package', () => {
+      // This would normally load 'class-transformer' package
+      // For testing, we pass a mock transformer package
+      const options = {
+        transformerPackage: mockTransformerPackage,
+      };
+
+      interceptor = new ClassSerializerInterceptor(mockReflector, options);
+
+      expect(interceptor).to.be.instanceOf(ClassSerializerInterceptor);
+    });
+
+    it('should use provided transformer package from options', () => {
+      const customTransformer = {
+        classToPlain: sandbox.stub(),
+        plainToInstance: sandbox.stub(),
+      };
+
+      const options = {
+        transformerPackage: customTransformer,
+      };
+
+      interceptor = new ClassSerializerInterceptor(mockReflector, options);
+
+      expect(interceptor).to.be.instanceOf(ClassSerializerInterceptor);
+    });
+  });
+
+  describe('intercept', () => {
+    let mockExecutionContext: ExecutionContext;
+    let mockCallHandler: CallHandler;
+
+    beforeEach(() => {
+      interceptor = new ClassSerializerInterceptor(mockReflector, {
+        transformerPackage: mockTransformerPackage,
+      });
+
+      mockExecutionContext = {
+        getHandler: sandbox.stub(),
+        getClass: sandbox.stub(),
+      } as any;
+
+      mockCallHandler = {
+        handle: sandbox.stub(),
+      } as any;
+    });
+
+    it('should transform plain object response', done => {
+      const response = { id: 1, name: 'Test' };
+      const transformedResponse = { id: 1, name: 'Test' };
+
+      mockReflector.getAllAndOverride.returns(undefined);
+      mockTransformerPackage.classToPlain.returns(transformedResponse);
+      (mockCallHandler.handle as sinon.SinonStub).returns(of(response));
+
+      interceptor
+        .intercept(mockExecutionContext, mockCallHandler)
+        .subscribe(result => {
+          expect(result).to.equal(transformedResponse);
+          done();
+        });
+    });
+
+    it('should transform array of objects', done => {
+      const response = [
+        { id: 1, name: 'Test1' },
+        { id: 2, name: 'Test2' },
+      ];
+      const transformedItem1 = { id: 1, name: 'Test1' };
+      const transformedItem2 = { id: 2, name: 'Test2' };
+
+      mockReflector.getAllAndOverride.returns(undefined);
+      mockTransformerPackage.classToPlain
+        .onFirstCall()
+        .returns(transformedItem1);
+      mockTransformerPackage.classToPlain
+        .onSecondCall()
+        .returns(transformedItem2);
+      (mockCallHandler.handle as sinon.SinonStub).returns(of(response));
+
+      interceptor
+        .intercept(mockExecutionContext, mockCallHandler)
+        .subscribe(result => {
+          expect(result).to.be.an('array').with.lengthOf(2);
+          expect(result[0]).to.equal(transformedItem1);
+          expect(result[1]).to.equal(transformedItem2);
+          done();
+        });
+    });
+
+    it('should merge context options with default options', done => {
+      const response = { id: 1, name: 'Test' };
+      const defaultOptions = { excludeExtraneousValues: true };
+      const contextOptions = { groups: ['user'] };
+      const transformedResponse = { id: 1 };
+
+      interceptor = new ClassSerializerInterceptor(mockReflector, {
+        transformerPackage: mockTransformerPackage,
+        ...defaultOptions,
+      });
+
+      mockReflector.getAllAndOverride.returns(contextOptions);
+      mockTransformerPackage.classToPlain.returns(transformedResponse);
+      (mockCallHandler.handle as sinon.SinonStub).returns(of(response));
+
+      interceptor
+        .intercept(mockExecutionContext, mockCallHandler)
+        .subscribe(result => {
+          const callArgs = mockTransformerPackage.classToPlain.getCall(0).args;
+          expect(callArgs[1]).to.deep.include(defaultOptions);
+          expect(callArgs[1]).to.deep.include(contextOptions);
+          done();
+        });
+    });
+
+    it('should call reflector with handler and class', done => {
+      const response = { id: 1 };
+      const mockHandler = {};
+      const mockClass = {};
+
+      (mockExecutionContext.getHandler as sinon.SinonStub).returns(mockHandler);
+      (mockExecutionContext.getClass as sinon.SinonStub).returns(mockClass);
+      mockReflector.getAllAndOverride.returns(undefined);
+      mockTransformerPackage.classToPlain.returns(response);
+      (mockCallHandler.handle as sinon.SinonStub).returns(of(response));
+
+      interceptor
+        .intercept(mockExecutionContext, mockCallHandler)
+        .subscribe(() => {
+          expect(mockReflector.getAllAndOverride.calledOnce).to.be.true;
+          const args = mockReflector.getAllAndOverride.getCall(0).args;
+          expect(args[1]).to.deep.equal([mockHandler, mockClass]);
+          done();
+        });
+    });
+  });
+
+  describe('serialize', () => {
+    beforeEach(() => {
+      interceptor = new ClassSerializerInterceptor(mockReflector, {
+        transformerPackage: mockTransformerPackage,
+      });
+    });
+
+    it('should return primitive values unchanged', () => {
+      expect(interceptor.serialize('string' as any, {})).to.equal('string');
+      expect(interceptor.serialize(123 as any, {})).to.equal(123);
+      expect(interceptor.serialize(true as any, {})).to.equal(true);
+    });
+
+    it('should return null unchanged', () => {
+      expect(interceptor.serialize(null as any, {})).to.be.null;
+    });
+
+    it('should return undefined unchanged', () => {
+      expect(interceptor.serialize(undefined as any, {})).to.be.undefined;
+    });
+
+    it('should return StreamableFile unchanged', () => {
+      const streamableFile = new StreamableFile(Buffer.from('test'));
+      const result = interceptor.serialize(streamableFile as any, {});
+
+      expect(result).to.equal(streamableFile);
+      expect(mockTransformerPackage.classToPlain.called).to.be.false;
+    });
+
+    it('should transform plain object', () => {
+      const input = { id: 1, name: 'Test' };
+      const output = { id: 1 };
+
+      mockTransformerPackage.classToPlain.returns(output);
+
+      const result = interceptor.serialize(input, {});
+
+      expect(result).to.equal(output);
+      expect(mockTransformerPackage.classToPlain.calledOnce).to.be.true;
+    });
+
+    it('should transform array of objects', () => {
+      const input = [{ id: 1 }, { id: 2 }];
+      const output1 = { id: 1 };
+      const output2 = { id: 2 };
+
+      mockTransformerPackage.classToPlain.onFirstCall().returns(output1);
+      mockTransformerPackage.classToPlain.onSecondCall().returns(output2);
+
+      const result = interceptor.serialize(input, {});
+
+      expect(result).to.be.an('array').with.lengthOf(2);
+      expect(result[0]).to.equal(output1);
+      expect(result[1]).to.equal(output2);
+      expect(mockTransformerPackage.classToPlain.calledTwice).to.be.true;
+    });
+
+    it('should handle empty array', () => {
+      const input: any[] = [];
+
+      const result = interceptor.serialize(input, {});
+
+      expect(result).to.be.an('array').that.is.empty;
+      expect(mockTransformerPackage.classToPlain.called).to.be.false;
+    });
+  });
+
+  describe('transformToPlain', () => {
+    beforeEach(() => {
+      interceptor = new ClassSerializerInterceptor(mockReflector, {
+        transformerPackage: mockTransformerPackage,
+      });
+    });
+
+    it('should return falsy values unchanged', () => {
+      expect(interceptor.transformToPlain(null, {})).to.be.null;
+      expect(interceptor.transformToPlain(undefined, {})).to.be.undefined;
+      expect(interceptor.transformToPlain(0 as any, {})).to.equal(0);
+      expect(interceptor.transformToPlain(false as any, {})).to.be.false;
+      expect(interceptor.transformToPlain('' as any, {})).to.equal('');
+    });
+
+    it('should use classToPlain when no type option provided', () => {
+      const input = { id: 1, name: 'Test' };
+      const output = { id: 1 };
+
+      mockTransformerPackage.classToPlain.returns(output);
+
+      const result = interceptor.transformToPlain(input, {});
+
+      expect(result).to.equal(output);
+      expect(mockTransformerPackage.classToPlain.calledOnceWith(input, {})).to
+        .be.true;
+      expect(mockTransformerPackage.plainToInstance.called).to.be.false;
+    });
+
+    it('should use classToPlain when input is instance of options.type', () => {
+      class UserDto {
+        id: number;
+        name: string;
+      }
+
+      const input = new UserDto();
+      input.id = 1;
+      input.name = 'Test';
+
+      const output = { id: 1 };
+      const options = { type: UserDto };
+
+      mockTransformerPackage.classToPlain.returns(output);
+
+      const result = interceptor.transformToPlain(input, options);
+
+      expect(result).to.equal(output);
+      expect(mockTransformerPackage.classToPlain.calledOnce).to.be.true;
+      expect(mockTransformerPackage.plainToInstance.called).to.be.false;
+    });
+
+    it('should convert plain to instance then to plain when type provided but not matching', () => {
+      class UserDto {
+        id: number;
+      }
+
+      const plainInput = { id: 1, name: 'Test', password: 'secret' };
+      const instanceOutput = new UserDto();
+      instanceOutput.id = 1;
+
+      const finalOutput = { id: 1 };
+      const options = { type: UserDto };
+
+      mockTransformerPackage.plainToInstance.returns(instanceOutput);
+      mockTransformerPackage.classToPlain.returns(finalOutput);
+
+      const result = interceptor.transformToPlain(plainInput, options);
+
+      expect(result).to.equal(finalOutput);
+      expect(
+        mockTransformerPackage.plainToInstance.calledOnceWith(
+          UserDto,
+          plainInput,
+          options,
+        ),
+      ).to.be.true;
+      expect(
+        mockTransformerPackage.classToPlain.calledOnceWith(
+          instanceOutput,
+          options,
+        ),
+      ).to.be.true;
+    });
+
+    it('should handle complex nested objects', () => {
+      const input = {
+        user: { id: 1, name: 'Test' },
+        posts: [{ id: 1, title: 'Post 1' }],
+      };
+      const output = {
+        user: { id: 1 },
+        posts: [{ id: 1 }],
+      };
+
+      mockTransformerPackage.classToPlain.returns(output);
+
+      const result = interceptor.transformToPlain(input, {});
+
+      expect(result).to.equal(output);
+    });
+  });
+
+  describe('getContextOptions', () => {
+    beforeEach(() => {
+      interceptor = new ClassSerializerInterceptor(mockReflector, {
+        transformerPackage: mockTransformerPackage,
+      });
+    });
+
+    it('should call reflector getAllAndOverride with correct arguments', () => {
+      const mockHandler = {};
+      const mockClass = {};
+      const mockContext = {
+        getHandler: sandbox.stub().returns(mockHandler),
+        getClass: sandbox.stub().returns(mockClass),
+      } as any;
+
+      const expectedOptions = { groups: ['admin'] };
+      mockReflector.getAllAndOverride.returns(expectedOptions);
+
+      const result = (interceptor as any).getContextOptions(mockContext);
+
+      expect(mockReflector.getAllAndOverride.calledOnce).to.be.true;
+      const callArgs = mockReflector.getAllAndOverride.getCall(0).args;
+      expect(callArgs[1]).to.deep.equal([mockHandler, mockClass]);
+      expect(result).to.equal(expectedOptions);
+    });
+
+    it('should return undefined when no metadata exists', () => {
+      const mockContext = {
+        getHandler: sandbox.stub().returns({}),
+        getClass: sandbox.stub().returns({}),
+      } as any;
+
+      mockReflector.getAllAndOverride.returns(undefined);
+
+      const result = (interceptor as any).getContextOptions(mockContext);
+
+      expect(result).to.be.undefined;
+    });
+
+    it('should respect handler metadata over class metadata', () => {
+      const mockHandler = {};
+      const mockClass = {};
+      const mockContext = {
+        getHandler: sandbox.stub().returns(mockHandler),
+        getClass: sandbox.stub().returns(mockClass),
+      } as any;
+
+      // getAllAndOverride should merge with handler taking precedence
+      const handlerOptions = {
+        groups: ['user'],
+        excludeExtraneousValues: true,
+      };
+      mockReflector.getAllAndOverride.returns(handlerOptions);
+
+      const result = (interceptor as any).getContextOptions(mockContext);
+
+      expect(result).to.deep.equal(handlerOptions);
+      // Verify it's checking handler first, then class
+      const callArgs = mockReflector.getAllAndOverride.getCall(0).args;
+      expect(callArgs[1][0]).to.equal(mockHandler);
+      expect(callArgs[1][1]).to.equal(mockClass);
+    });
+  });
+
+  describe('edge cases and error handling', () => {
+    beforeEach(() => {
+      interceptor = new ClassSerializerInterceptor(mockReflector, {
+        transformerPackage: mockTransformerPackage,
+      });
+    });
+
+    it('should handle array with mixed types', () => {
+      const input = [
+        { id: 1, name: 'Test' },
+        null,
+        undefined,
+        { id: 2, name: 'Test2' },
+      ];
+
+      mockTransformerPackage.classToPlain
+        .onCall(0)
+        .returns({ id: 1, name: 'Test' });
+      mockTransformerPackage.classToPlain.onCall(1).returns(null);
+      mockTransformerPackage.classToPlain.onCall(2).returns(undefined);
+      mockTransformerPackage.classToPlain
+        .onCall(3)
+        .returns({ id: 2, name: 'Test2' });
+
+      const result = interceptor.serialize(input, {});
+
+      expect(result).to.be.an('array').with.lengthOf(4);
+      expect(result[1]).to.be.null;
+      expect(result[2]).to.be.undefined;
+    });
+
+    it('should not transform when response is not an object', () => {
+      const input = 'plain string';
+
+      const result = interceptor.serialize(input as any, {});
+
+      expect(result).to.equal(input);
+      expect(mockTransformerPackage.classToPlain.called).to.be.false;
+    });
+
+    it('should handle Date objects', () => {
+      const date = new Date();
+      const output = { date: date.toISOString() };
+
+      mockTransformerPackage.classToPlain.returns(output);
+
+      const result = interceptor.serialize({ date } as any, {});
+
+      expect(mockTransformerPackage.classToPlain.calledOnce).to.be.true;
+    });
+
+    it('should pass through options to transformer', () => {
+      const input = { id: 1, name: 'Test', password: 'secret' };
+      const options = {
+        excludeExtraneousValues: true,
+        groups: ['public'],
+        strategy: 'excludeAll',
+      };
+
+      mockTransformerPackage.classToPlain.returns({ id: 1, name: 'Test' });
+
+      interceptor.transformToPlain(input, options as any);
+
+      expect(mockTransformerPackage.classToPlain.calledOnce).to.be.true;
+      const callArgs = mockTransformerPackage.classToPlain.getCall(0).args;
+      expect(callArgs[1]).to.deep.include(options);
+    });
+  });
+});