I copied following code snippet from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleDateString and tried it in my own project.

const event = new Date(Date.UTC(2012, 11, 20, 3, 0, 0));
const options = { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' };
console.log(event.toLocaleDateString('de-DE', options));

However I get this error on my parameter 'options':

Argument of type '{ weekday: string; year: string; month: string; day: string; }' is not assignable to parameter of type 'DateTimeFormatOptions'. Types of property 'weekday' are incompatible. Type 'string' is not assignable to type '"long" | "short" | "narrow" | undefined'.

What is the problem here?

CodePudding user response：

Desired type of weekday is: "long" | "short" | "narrow" | undefined

As you can see, it is not a string type - it is a union of predefined constants (or undefined).

Meanwhile, your options is inferred to the following type:

{
    weekday: string;
    year: string;
    month: string;
    day: string;
}

You need to prevent widening your literals to string.

There are 3 ways to do that:

• as constassertion
• explicit type for options variable
• inlining the variable

// Option 1: as const assetion
const options = { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' } as const;
// inferred to:
// { 
// readonly weekday: "long";
// readonly year: "numeric";
// readonly month: "long";
// readonly day: "numeric";
//};
console.log(ee.toLocaleDateString('de-DE', options));

// Option 2: Explicit type
const options2:  Intl.DateTimeFormatOptions = { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' };
console.log(ee.toLocaleDateString('de-DE', options2));

// Option 3: inlining options
console.log(ee.toLocaleDateString('de-DE', { weekday: 'long', year: 'numeric', month: 'long', day: 'numeric' }));

Playground link