This pull request addresses the issue raised in #6202, where the current TypeScript definitions for the toJSON method do not accurately reflect the possible return value of null for invalid date instances. As documented, the toJSON method returns a string representation of the moment object when valid and null when the moment object represents an invalid date. However, the existing TypeScript type definitions only indicate a string return type, leading to potential type mismatches and runtime errors in TypeScript applications.

Changes Made

• Updated the TypeScript definitions for the toJSON method to include null as a possible return type. The method now correctly reflects its return type as string | null, aligning with the actual behavior of the toJSON method.

Impact

• This change ensures that TypeScript users of moment.js have accurate type information, preventing type-related runtime errors when dealing with invalid date instances. It reinforces the reliability of TypeScript definitions in the moment.js library.

Testing

• Added new test to verify that TypeScript correctly recognizes the updated return type of the toJSON method. This test ensure that no type errors occur when a moment object is invalid, and toJSON returns null.
• All existing tests have been run to ensure that this change does not affect other functionalities or introduce regressions.

Additional Notes

• This change is focused solely on updating the TypeScript definitions to match the current JavaScript implementation of the toJSON method. No changes were made to the method's implementation.

By accurately reflecting the toJSON method's behavior in the TypeScript definitions, this pull request aims to improve the development experience for TypeScript users of moment.js, ensuring type safety and consistency with the library's runtime behavior.

Resolves #6202.