Typescript: Type 'string' is not assignable to type '"numeric" | "2-digit"' in Date::toLocaleDateString()

Question

I have recently been alerted to an "error" by Visual Studio Code in the following snippet:

someDateObject.toLocaleDateString('de-DE', Travel.dateOptions));
someDateObject.toLocaleDateString('de-DE', Travel.dateOptions));

Where Travel.dateOptions is defined as such:

public static dateOptions = { year: 'numeric', month: '2-digit', day: '2-digit' };

This has been working fine for the better part of the last 2 years, but upon opening the class inside VSC recently, it displayed following error for Travel.dateOptions:

Argument of type '{ year: string; month: string; day: string; }' is not assignable to parameter of 
type 'DateTimeFormatOptions'.
Types of property 'year' are incompatible.
Type 'string' is not assignable to type '"numeric" | "2-digit"'. ts(2345)

I am dead confused as to why. Is this possibly a bug with VSC? The code seems to work fine (and has worked fine the entire time) once compiled - and according to the documentation for Date::toLocaleDateString() what I'm doing here seems perfectly valid.

Any ideas?

Answer 1

When you initialize a class property with a literal such as public foo = { bar: 'a' }, its type becomes { bar: string }, even if you declare it as readonly. TypeScript on purpose doesn't make the type too strict ({ bar: 'a' }).

Method toLocaleDateString accepts an object whose key year must be of type 'numeric' or '2-digit', but yours is of type string.

To make the type of the initialized object more specific, use as const:

public static dateOptions = { year: 'numeric', month: '2-digit', day: '2-digit' } as const;