in the example below, i want to assign a variable like i do to x1 and have it be type C. i can accomplish this with the way i do x2 or presumably with as but that's clunky or unsafe. is there a nice way to do this? i'd like type C so that some type inference stuff later down the line works right.

type A = {foo: "foo"}
type B = {bar: "bar"}

type C = A | B

let x1: C = {foo: "foo"}
console.log(x1) // <- x1 is type A (what i want is for it to be C)

let x2: C = ((): C => ({foo: "foo"}))()
console.log(x2) // <- x2 is type C (desired)

ts playground link

CodePudding user response：

This looks like it may be a case for a type assertion using as:

let x1: C = {foo: 'foo'} as C;

Though I'm not sure what use case might require TypeScript to not know what specific type of C value you've assigned to x1?

With the way you have it currently, TypeScript does know that x1 is of type C. You can see this by trying to assign it a value of type B later, for example, and it will still work, though in that case TypeScript will still know you've given it a value of type B.

let x1: C = {foo: "foo", bar: 'bar'};
console.log(x1) // <- x1 is type A (what i want is for it to be C)

x1 = {bar: 'bar'};
console.log(x1) // <- TypeScript knows x1 now holds a value of type B

You can also see that if you assign it a type that matches A & B then TypeScript will not try to narrow its type any further than C:

let x1: C = {foo: 'foo', bar: 'bar'};