Day.js 1.11.19 TypeScript Regression: Year() Type Mismatch

Nov 17, 2025 by Alex Johnson 59 views

After upgrading Day.js from version 1.11.10 to 1.11.19, a TypeScript type error surfaced, specifically related to the year() method. This issue indicates a potential regression or unintended breaking change within the patch releases. This article provides an in-depth look at the problem, its context, and potential solutions for developers encountering this issue.

Understanding the Issue

The core problem lies in a TypeScript type mismatch that arises when using the year() method after upgrading Day.js. In essence, the TypeScript compiler flags an incompatibility between the types returned by value.year(...). The error message explicitly states that a number type is not assignable to a Dayjs type, which is unexpected since, in previous versions, year() reliably returned a numerical representation of the year. This change disrupts existing codebases that rely on the previous behavior, causing build failures and potentially affecting application functionality.

Error Message Details

The precise error message observed is:

The types returned by 'value.year(...)' are incompatible between these types.
  Type 'number' is not assignable to type 'Dayjs'.

This error indicates a significant change in the type definitions within Day.js, specifically affecting how the year() method is typed. Developers expect year() to return a number, but the updated type definitions seem to be returning a Dayjs object instead. Such a discrepancy necessitates a careful review of the changes introduced in Day.js 1.11.19 and subsequent versions to identify the root cause.

Context: React + Vite Project

It's important to note that this issue was observed within a React project using Vite as the build tool. While the problem is directly related to Day.js type definitions, the surrounding environment might influence how the error manifests. Vite's configuration and TypeScript settings could interact with Day.js types in specific ways, potentially exacerbating the issue or revealing nuances that might not be apparent in other project setups. Therefore, examining the Vite configuration and TypeScript settings is crucial for a comprehensive understanding.

Affected Versions

The following versions are known to be relevant:

Working Day.js version: 1.11.10
Failing Day.js version: 1.11.19
TypeScript version: 5.5.4
React version: 18.2.0

The transition from Day.js 1.11.10 to 1.11.19 introduced the type mismatch. TypeScript version 5.5.4 is used for type checking, and React 18.2.0 forms the front-end framework. Identifying the precise changes between Day.js 1.11.10 and 1.11.19 is critical to pinpointing the source of the regression. Examining the Day.js changelog, commit history, and type definition files can help isolate the specific modification that led to this type error. This investigation may require a deep dive into the Day.js codebase and an understanding of how type definitions are generated and managed within the library.

Potential Causes and Solutions

Given the information, several potential causes could explain the observed regression. Each potential cause suggests different diagnostic and resolution strategies.

Root Cause Analysis

Accidental Type Definition Change: A likely cause is an unintended change in the TypeScript definition files (.d.ts) within Day.js. This could result from a refactoring effort, a bug fix that inadvertently altered the return type of the year() method, or a mistake in the type definition generation process. Examining the diff between the type definition files of Day.js 1.11.10 and 1.11.19 is crucial to identifying any such changes.
TypeScript Version Incompatibility: It is also possible, though less likely given the TypeScript version, that changes in Day.js 1.11.19 rely on newer TypeScript features or behaviors that are not fully compatible with TypeScript 5.5.4. While TypeScript generally aims for backward compatibility, subtle changes in type inference or checking could trigger unexpected errors.
Build Configuration Issues: Misconfigurations in the build process, particularly within Vite, could lead to incorrect type resolution or the inclusion of outdated type definition files. This is less likely if the upgrade was straightforward, but it is worth investigating if other solutions fail.

Workaround Strategies

Downgrade Day.js Version: The simplest workaround is to revert to Day.js version 1.11.10, the last known working version. While this resolves the immediate compilation error, it also means missing out on any bug fixes or new features introduced in later Day.js versions. Downgrading should be considered a temporary solution while a more permanent fix is investigated.
Type Assertion: Another workaround involves using type assertions in your code to explicitly tell the TypeScript compiler that the year() method returns a number. This can be done using the as keyword or the non-null assertion operator (!). However, this approach should be used cautiously, as it bypasses TypeScript's type checking and could mask underlying issues.
```
const yearValue = value.year() as number;
// or
const yearValue = value.year()!;
```
While type assertions can silence the error, they do not address the root cause and might lead to runtime errors if the actual return type is not a number.
Patching Day.js: For more advanced users, patching the Day.js type definition files directly is an option. This involves modifying the .d.ts files within the node_modules/dayjs directory to correct the return type of the year() method. However, this approach is fragile, as any subsequent Day.js updates will overwrite the changes. It also requires a good understanding of TypeScript type definitions and the Day.js codebase.

Contributing to the Solution

Report the Issue: If you encounter this issue, reporting it to the Day.js maintainers is essential. Provide a clear and concise description of the problem, including the error message, affected versions, and a minimal reproducible example. This will help the maintainers understand the issue and prioritize a fix.
Investigate the Code: If you have the time and expertise, consider investigating the Day.js codebase to identify the root cause of the regression. Examine the commit history, type definition files, and related code to pinpoint the change that introduced the type mismatch. Sharing your findings with the Day.js maintainers can significantly expedite the resolution process.
Test Beta Versions: Keep an eye out for beta or pre-release versions of Day.js that address this issue. Testing these versions and providing feedback to the maintainers can help ensure that the fix is effective and does not introduce new problems.

Detailed Steps to Resolve the Issue

Step-by-Step Guide

Verify the Issue: Ensure that the type error indeed stems from the Day.js upgrade. Double-check the error message and the affected code to confirm that the year() method is the culprit.
Examine Day.js Changelog: Review the Day.js changelog for versions between 1.11.10 and 1.11.19. Look for any changes that might relate to type definitions, method signatures, or return types. The changelog might provide clues about the cause of the regression.
Inspect Type Definition Files: Compare the type definition files (.d.ts) for Day.js 1.11.10 and 1.11.19. Use a diff tool to highlight the differences between the files, focusing on the definition of the year() method. This comparison can reveal any unintended changes in the return type.
Test with Different TypeScript Versions: Try compiling your project with different TypeScript versions to see if the issue is specific to TypeScript 5.5.4. You can use tools like nvm or asdf to manage multiple Node.js and TypeScript versions.
Implement a Workaround: Choose a suitable workaround based on your project's needs and constraints. Downgrading Day.js is the safest option, while type assertions can provide a quick fix. Patching Day.js should only be considered as a last resort.
Report the Issue: If the problem persists, report the issue to the Day.js maintainers with detailed information about your environment, the error message, and any steps you have taken to investigate the issue.

Practical Example

Consider the following code snippet that triggers the type error:

import dayjs from 'dayjs';

const now = dayjs();
const year = now.year(); // TypeScript error: Type 'Dayjs' is not assignable to type 'number'.

console.log(year);

To resolve this issue using a type assertion, you can modify the code as follows:

import dayjs from 'dayjs';

const now = dayjs();
const year = now.year() as number; // Type assertion to resolve the error.

console.log(year);

While this resolves the compilation error, it is essential to remember that the underlying issue remains. A proper fix would involve correcting the type definition files in Day.js or addressing any incompatibilities with TypeScript.

Conclusion

The TypeScript type mismatch encountered after upgrading Day.js from 1.11.10 to 1.11.19 is a significant issue that can disrupt development workflows. By understanding the potential causes, implementing appropriate workarounds, and actively contributing to the resolution process, developers can mitigate the impact of this regression and ensure the stability of their projects. Remember to always thoroughly test upgrades and monitor for any unexpected issues that might arise.

For more information about Day.js, visit the official Day.js documentation. This resource provides comprehensive information about the library's features, API, and usage.