Date.prototype.toJSON has incorrect return type

Bug Report

Date.prototype.toJSON() can return a string or null according to the spec https://tc39.es/ecma262/#sec-date.prototype.tolocaledatestring. For example new Date("").toJSON() returns null.

Currently, the return type of toJSON() is specified as a string:
https://github.com/microsoft/TypeScript/blob/131875bb849c0a9c56c26d09881453aace0cbfed/lib/lib.es5.d.ts#L879

🕗 Version & Regression Information

Typescript 4.2.3

💻 Code

new Date("").toJSON().slice(0,5);

produces an error at runtime since new Date("").toJSON() is null, but doesn't produce a typescript error because new Date("").toJSON() is assumed to be string.

🙂 Expected behavior

TypeScript should warn about the potentially null value when an arbitrary string is passed into Date. However, if the Date object can be guaranteed to be valid (for example new Date()), the return type of toJSON() should be string.

lib/lib.es5.d.ts の Date.prototype.toJSON の宣言を確認し、無効な Date と new Date() を使って提供された例を実行してください。TypeScript の型が結果が null になる可能性を警告しつつ、有効性が保証されている場合は string の結果を維持すれば、issue は完了です。