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.

Read the declaration for Date.prototype.toJSON in lib/lib.es5.d.ts, then run the supplied example with an invalid Date and with new Date(). The issue is done when the TypeScript types warn about the potentially null result while preserving a string result when validity is guaranteed.