[SOLVED] Extract actual object keys from typed Partial object, instead of all possible keys

Extract actual object keys from typed Partial object, instead of all possible keys

I am trying to work out how to extract a list of keys from an object, while at the same time limiting those keys so they belong to a certain superset of values.

I can either get the list of keys, OR limit the values to an allowed list, but I cannot work out how to do both at the same time.

For example:

// If I don't specify the type of the keys, then I can extract them easily.
const workdayAttributesAuto = {
    Monday: 123,
    Tuesday: 456,
    Floofday: -1,  // whoops, I can specify an invalid day though
};

// Now I can extract them to another type just fine.  Here only Monday
// and Tuesday are included, but I also get the unwanted invalid value.
export type workdayNamesAuto = keyof typeof workdayAttributesAuto;
// --> type workdayNamesAuto = 'Monday' | 'Tuesday' | 'Floofday';

So to avoid the invalid value, I can specify a type to restrict keys to only a list of allowed values. But in doing so, I can no longer extract the actual keys used, and I end up with all permitted keys instead:

type Days = 'Sunday' | 'Monday' | 'Tuesday'; // only these keys are allowed

const workdayAttributes: Partial<Record<Days, number>> = {
    // Sunday is intentionally omitted.
    Monday: 123,
    Tuesday: 456,
    //Floofday: 0,  // not allowed to specify invalid days, good
};

// But now the same type includes Sunday, when it wasn't one of the object keys.
export type workdayNames = keyof typeof workdayAttributes;
// --> type workdayNames2 = 'Sunday' | 'Monday' | 'Tuesday';

How can I tell keyof typeof to only get the keys actually specified in the object, instead of it getting all possible allowed values?

Solution

Use the new satisfies operator, new in TS 4.9:

type Days = 'Sunday' | 'Monday' | 'Tuesday'; // only these keys are allowed

const workdayAttributes = {
    // Sunday is intentionally omitted.
    Monday: 123,
    Tuesday: 456,
    // Floofday: 0,  // not allowed to specify invalid days
} satisfies Partial<Record<Days, number>>;

export type workdayNames = keyof typeof workdayAttributes;
// --> type workdayNames = 'Monday' | 'Tuesday';