TypeScript Testing
Master TypeScript testing — Vitest, type testing, mocking with type safety, and testing patterns for TypeScript applications.
Testing TypeScript Code
- Vitest/Jest: TypeScript-native test runner — import { describe, it, expect } from 'vitest'. Direct TS execution without separate compilation step. ESM support out of the box
- Type Testing: expectTypeOf<ReturnType<typeof createUser>>().toEqualTypeOf<User>() — vitest type tests verify your types are correct. Catch type regressions in CI
- Mocking: vi.mock('./database') with typed mocks — MockedFunction<typeof fetchUser> ensures mock return types match real function. No runtime type mismatches in tests
- Test Utilities: Partial<User> for test data factories — function createTestUser(overrides?: Partial<User>): User { return { ...defaults, ...overrides } }. Type-safe test data creation
- Integration Testing: supertest with typed responses — const response = await request(app).get('/users'); expect(response.body as User[]).toHaveLength(2). End-to-end type safety
- Property-Based Testing: fast-check with TypeScript — fc.record({ name: fc.string(), age: fc.nat() }) generates random inputs matching your types. Finds edge cases human-written tests miss
Try It Yourself: Advanced TypeScript
Line 24: Malformed tag: "<\"hello\">"
💡 Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">
Line 24: Malformed tag: "<42>"
💡 Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">
Line 24: Malformed tag: "<() =>"
💡 Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">
Line 25: Unknown tag <user> on line 25
💡 "<user>" is not a valid HTML5 element. Use <div>, <span>, <p>, <section>, etc.
Line 29: Wrong closing tag: </script> found, expected </user> (line 29)
💡 Close <user> (opened on line 25) with </user> before closing </script>.
Line 29: Wrong closing tag: </body> found, expected </user> (line 29)
💡 Close <user> (opened on line 25) with </user> before closing </body>.
Line 29: Wrong closing tag: </html> found, expected </user> (line 29)
💡 Close <user> (opened on line 25) with </user> before closing </html>.
Line 2: Unclosed <html> (opened on line 2)
💡 Add </html> to properly close this element.
Line 11: Unclosed <body> (opened on line 11)
💡 Add </body> to properly close this element.
Line 21: Unclosed <script> (opened on line 21)
💡 Add </script> to properly close this element.
Line 25: Unclosed <user> (opened on line 25)
💡 Add </user> to properly close this element.