"TS2322: Type 'Timeout' is not assignable to type 'number'" when running unit tests

ONLINECAST

3 min read

·

05-09-2024

16

0

Share

Unraveling "TS2322: Type 'Timeout' is not assignable to type 'number'" in TypeScript Unit Tests

Running unit tests is a cornerstone of robust software development. However, encountering TypeScript errors like "TS2322: Type 'Timeout' is not assignable to type 'number'" can be frustrating, especially when using npm link to manage dependencies. This article will delve into the root cause of this error, exploring the conflicting definitions of setTimeout and providing solutions to ensure your tests run smoothly.

Understanding the Error

This error occurs when TypeScript's type checker encounters a mismatch between the expected type (number) and the actual type of the return value from setTimeout, which is Timeout. This mismatch arises from the use of different definitions of setTimeout, one from the browser environment (typescript/lib/lib.dom) and the other from Node.js (@types/node/index).

Here's a breakdown of the situation:

• Browser Environment: In a browser, setTimeout is typically defined within the Window interface, returning a Timeout object representing a timer.
• Node.js Environment: In Node.js, setTimeout is defined in the global scope, returning a number that represents a timer identifier.

The problem surfaces when npm link is used to link Package A (which relies on a browser-centric definition of setTimeout) to Package B. The use of npm link allows Package A to access Package B's dependencies, potentially introducing Node.js-specific definitions that clash with the browser definitions expected in Package A's code.

Troubleshooting and Solutions

1. Identifying the Source of the Conflict

• Inspect type definitions: Use your IDE's type hinting or tools like tsc --showConfig to examine the definitions being used for setTimeout in both Package A and Package B.
• Verify dependencies: Ensure the versions of @types/node and @types/window (or similar browser-related types) in both packages are compatible and aligned with your project's needs.

2. Resolving the Type Conflict

• Explicit Type Casting: Use type casting to explicitly tell the TypeScript compiler the expected type of the setTimeout return value. For example:

  const timeoutId = setTimeout(() => {
    // Your code here
  }, 1000) as number; 
  

  This approach is a quick fix, but it might mask potential underlying issues with your code's dependency management.

• Use globalThis: Access setTimeout directly from the globalThis object, which represents the global scope in both Node.js and browser environments. This approach is often preferred when you need to access global functions within the context of your unit tests.

  const timeoutId = globalThis.setTimeout(() => {
    // Your code here
  }, 1000);
  

• Refactor Your Code: If possible, consider refactoring your code to abstract the use of setTimeout into a separate function or class. This will make it easier to manage different implementations of setTimeout based on the environment.

3. Utilize a Mocking Library

• Mocking frameworks like Jest or Sinon.js allow you to control the behavior of setTimeout within your unit tests, eliminating the need to rely on its actual implementation. This approach offers a more reliable and testable solution, especially in complex scenarios.

4. Address Dependency Management

• Use a package manager: If you're encountering this issue while using npm link, consider using a more robust package management solution like yarn or pnpm. These tools often provide better handling of dependencies and their respective types.
• Maintain consistent dependencies: Ensure both Package A and Package B are using the same versions of @types/node and other relevant type packages. This will minimize the likelihood of encountering type conflicts due to version discrepancies.

5. Choose the Correct Environment

• Target a specific environment: If your code is strictly targeted for the browser or Node.js, explicitly configure your testing environment to match. This will prevent mismatches in type definitions.

Conclusion

The "TS2322: Type 'Timeout' is not assignable to type 'number'" error is often a symptom of conflicting definitions of setTimeout arising from dependency management practices. By understanding the underlying cause and implementing the solutions outlined above, you can ensure your TypeScript code runs smoothly in various environments and test scenarios. Remember, robust testing is key to building reliable software.

Original Stack Overflow question by user "David" at: https://stackoverflow.com/questions/68609013/ts2322-type-timeout-is-not-assignable-to-type-number-when-running-unit-tests