Support 'as const' together with type restriction

[KnutRyagerInmeta]

Would be nice to be able to apply type checking to 'as const' types, currently I have to use the silly trick of using type checks when writing them, then switch to 'as const' afterwards.

type MyType = {name: string};

const x:MyType = {
    name: 'test' // Autocompleted, typesafe. But Type is {name: string}, not 
                 // what I want, I want {readonly name: 'test'}
} as const;

const x = { name: 'test' } as const; // Gives correct type, but no type check for MyType...

[MartinJohns]

Your type MyType clearly says that name is a mutable property. If you want it read-only, then you can just use Readonly<MyType>.

[jcalz]

Helper function to the rescue!

type MyType = { name: string };
const asMyType = <T extends MyType>(x: T) => x; // helper function
const x = asMyType({ name: "test" } as const); // 😃

[KnutRyagerInmeta]

@jcalz this does the trick.

@MartinJohns readonly was not originally of my concern, but emerged with 'as const'. Seems with jcalz's solution readony does not emerge. Now a single string literal type should be readonly by implication though... well you are allowed to overwrite with the same string I guess.

[MartinJohns]

@KnutRyagerInmeta You mean a string literal type, e.g. 'fixed' instead of string. In that case the same logic applies: Your type MyType explicitly says the property name can be of any string.

With jcalz solution you will have the same issue when you use the wrong type:

const x: MyType = asMyType({ name: "test" } as const); // x.name is of type string again

But I likely misunderstand you, and you want something like "make sure this object matches SomeType, then make it a const context", in which case jcalz solution is probably the best.