While working on test cases for a NestJS application, I encountered a peculiar TypeScript error that halted my progress:
๐จ Error Message:
_Type ‘Promise<{ userName: string; }>’ is not assignable to type ‘Promise<(Document<unknown, {}, UserDoc> & UserDoc & Required<{ id: unknown; }>)[]>.’
This error was not only unexpected but also a bit frustrating at first. It highlighted a type mismatch issue in one of my Jest tests, specifically when using jest.spyOn. If you’ve worked with TypeScript and testing frameworks like Jest, you know how common type issues can be when dealing with mocked functions.
In this article, I’ll walk you through what caused this error, the steps I took to troubleshoot it, and how I ultimately resolved it by properly typing jest.spyOn.
Understanding the Error
What Does the Error Mean?
The error essentially points to a type mismatch between the value being returned by a mocked function and the type expected by TypeScript. Here’s the critical part of the error:
_Type ‘Promise<{ userName: string; }>’ is not assignable to type ‘Promise<(Document<unknown, {}, UserDoc> & UserDoc & Required<{ id: unknown; }>)[]>.’
Breaking this down:
Promise<{ userName: string; }>: This is the type of the mocked value being returned by the function.Promise<(Document<unknown, {}, UserDoc> & UserDoc & Required<{ _id: unknown; }>)[]>: This is the type TypeScript expects based on the original function's implementation.
The issue arises because the mocked function is returning a simple object ({ userName: string }), whereas TypeScript expects a more complex type (Document objects with additional metadata).
What Caused the Error?
This type mismatch typically occurs when:
- Using Jest’s
spyOnfor Mocking: Jest’sspyOnallows you to mock specific methods of a class or object. However, TypeScript can’t infer the return type of the mocked method unless you explicitly define it. - TypeScript’s Strict Typing: By default, TypeScript enforces strict type checks, which means any deviation from the expected type will throw an error.
- Complex Return Types: If the function being mocked returns a complex type (e.g., a Mongoose
Document), TypeScript requires the mocked return value to match the exact type, even in tests.
The Solution: Adding a Type to jest.spyOn
After some research, I discovered that the key to resolving this error was explicitly typing jest.spyOn. By specifying the type for spyOn, I was able to tell TypeScript what to expect, thus avoiding the type mismatch.
Here’s the updated code that resolved the issue:
Before (Error):
jest.spyOn(userService, 'findUserById').mockResolvedValue({ userName: 'John Doe' });After (Fixed):
jest.spyOn<any, string>(userService, 'findUserById').mockResolvedValue({ userName: 'John Doe' });The addition of <any, string> explicitly defines the type of spyOn. Here’s what it means:
any: Specifies the type of the object being spied on. In this case, it’s theuserServiceinstance.string: Specifies the type of the method name being mocked. This ensures that TypeScript understands which method we’re referring to.
Why This Solution Works
By explicitly typing jest.spyOn, TypeScript no longer attempts to infer the type of the mocked method. Instead, it uses the type you’ve defined, allowing you to bypass the type mismatch. This approach works particularly well when:
- The mocked method has a complex return type.
- TypeScript’s inference isn’t sufficient to resolve the correct types.
Step-by-Step Debugging Process
1. Identify the Problematic Code
The first step was pinpointing the exact line of code causing the error. In this case, it was the jest.spyOn call mocking the findUserById method in the userService.
2. Understand the Expected Type
Next, I examined the implementation of findUserById in userService. The method was returning a Mongoose Document type, which explained why TypeScript expected a more complex type.
3. Research the Solution
I searched for similar issues in Jest and TypeScript forums and documentation. That’s when I came across the recommendation to explicitly type jest.spyOn.
4. Apply and Test the Fix
Finally, I updated the jest.spyOn call with the appropriate type parameters (<any, string>). Running the tests again confirmed that the error was resolved.
Lessons Learned
1. TypeScript Requires Precision
Even in test cases, TypeScript enforces type safety to ensure consistency. While this can be frustrating at times, it ultimately leads to more reliable code.
2. Explicit Typing is Powerful
When TypeScript’s inference isn’t enough, adding explicit types can save you from type mismatch errors. Tools like jest.spyOn<any, string> are invaluable in such scenarios.
3. Stay Updated with Tools
This issue reminded me of how important it is to stay up-to-date with changes in tools and frameworks. Both Jest and TypeScript evolve constantly, and keeping up with their best practices can save you hours of debugging.
Conclusion
This experience highlighted the challenges and rewards of working with TypeScript in a modern testing framework like Jest. While type errors can be a stumbling block, they also push you to write more robust and maintainable code.
If you’re working with Jest and encounter similar type errors, remember that explicitly typing jest.spyOn can often resolve the issue. And, as always, embrace the debugging process—it’s an opportunity to learn and grow as a developer.
Mission accomplished! Happy testing! ๐
Let me know if you’ve faced similar issues or if you have alternative solutions to share!