typescript-testing

name	typescript-testing
description	Comprehensive testing guidance for TypeScript projects including unit testing patterns, mocking strategies, and test organization best practices
version	1.0.0
priority	25
tags	["testing","typescript","vitest","jest","builtin"]
triggers	[{"type":"keyword","pattern":"test"},{"type":"keyword","pattern":"vitest"},{"type":"keyword","pattern":"jest"},{"type":"glob","pattern":"**/*.test.ts"},{"type":"glob","pattern":"**/*.spec.ts"}]
globs	["**/*.test.ts","**/*.spec.ts","**/vitest.config.ts","**/jest.config.ts"]

TypeScript Testing

Guidance for writing effective tests in TypeScript projects using Vitest or Jest.

Rules

• Arrange-Act-Assert: Structure tests with clear setup, execution, and verification phases
• Single Assertion Focus: Each test should verify one specific behavior
• Descriptive Names: Use describe blocks for grouping and it/test with clear descriptions
• Test Isolation: Tests must not depend on execution order or shared mutable state
• Mock External Dependencies: Always mock I/O, network calls, and system interactions
• Coverage Targets: Aim for 80%+ line coverage, 70%+ branch coverage
• No Logic in Tests: Avoid conditionals and loops in test code
• Clean Up: Use beforeEach/afterEach for setup/teardown, never leave test artifacts

Patterns

Test Structure

import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
import { MyService } from "./my-service.js";

describe("MyService", () => {
  let service: MyService;

  beforeEach(() => {
    service = new MyService();
    vi.useFakeTimers();
  });

  afterEach(() => {
    vi.useRealTimers();
    vi.restoreAllMocks();
  });

  describe("methodName", () => {
    it("should return expected result when given valid input", () => {
      // Arrange
      const input = "test";

      // Act
      const result = service.methodName(input);

      // Assert
      expect(result).toBe("expected");
    });

    it("should throw error when given invalid input", () => {
      expect(() => service.methodName(null)).toThrow("Invalid input");
    });
  });
});
typescript
 { vi,   }  ;
  {  }  ;


vi.(,  ({
  : vi.().( ({
    : vi.(),
    : vi.(),
  })),
}));


 mockFetch = vi.()  < fetch>;
vi.(, mockFetch);

mockFetch.(
   (.({ :  }), {
    : ,
    : { :  },
  })
);
typescript
(,  () => {
   promise = service.();
  
  
   vi.();
  
   result =  promise;
  (result).();
});

(,  () => {
  mockDependency.( ());
  
   (service.())..();
});
typescript
(,  {
   result = service.();
  (result).();
});

(,  {
   result = service.({ :  });
  (result).();
});
typescript

(,  {
   spy = vi.(service, );
  service.();
  (spy).(); 
});


(,  {
  (service.()).();
  (service.()).();
  (service.()).(); 
});


 : ;
(,  { sharedState = ; });
(,  { (sharedState).(); }); 


(,  {
  service.().( (r).()); 
});


mockFn.()
      .()
      .(); 
typescript
 { describe, it, expect, vi, beforeEach }  ;
 {  }  ;
  {  }  ;

(,  {
   : ;
   : ;

  ( {
    mockRepo = {
      : vi.(),
      : vi.(),
      : vi.(),
    };
    service =  (mockRepo);
  });

  (,  {
    (,  () => {
       user = { : , :  };
      vi.(mockRepo.).(user);

       result =  service.();

      (result).(user);
      (mockRepo.).();
    });

    (,  () => {
      vi.(mockRepo.).();

       result =  service.();

      (result).();
    });
  });
});
typescript
(,  {
  (,  () => {
     dbError =  ();
    vi.(mockRepo.).(dbError);

     (service.())..();
  });

  (,  {
    ( service.()).();
    ( service.()).();
  });
});

References

• Vitest Documentation
• Jest Documentation
• Testing Library
• MSW (Mock Service Worker)