如何在 Typescript 中选择特定的返回类型？

Question

Lets say return type of a function is a | b a | b . Now if I know that the return type will be a for sure for a particular call, how do I tell typescript to treat the returned value as a . Normally if I try this:

const x: a = func();

I get type a | b is not assignable to type a type a | b is not assignable to type a

Answer 1

Just cast the return type:

const x: a = <a>func();

In this way, you are telling typescript that you know for sure what the return type is.

---

Another way to do it is to create an overload for the function like so:

function func(): a;

Then call it like you did originally

const x: a = func();

Answer 2

Casting the particular value you're interested in is certainly a valid option, but it's based on a hidden assumption that only you, the programmer, know about. If 6 months down the line you change the behaviour of your function in such a way that your assumption is no longer valid, you'll get a runtime error.

A more robust solution would be to encode your assumption in the program through the type system, either via overloading or via conditional types.

For example:

function f1(x: string): number | string {
    return undefined;
}

const a1: number = f1('foo') // wrong, can't do

function f2(x: string): string
function f2(): number
function f2(x?: string): number | string {
    return undefined;
}

const a2s: string = f2('foo') // OK
const a2n: number = f2() // OK

Similarly, conditional types allow you to express the same thing:

function f3<T>(x?: T): T extends string ? string : number {
    return undefined;
}

const a3s: string = f3('foo') // OK
const a3n: number = f3()

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