Take a look at the code snippet below:
interface B<T> {
bla?: T;
}
class A implements B<string> {
}
type thisShouldBeString = A extends B<infer T> ? T : never;
I am trying to extract the string type from A. Is there a way to achieve this?
Edith: Here is an alternative method I attempted that failed to produce the desired result:
interface B<T> {
bla?: T;
}
class A implements B<string> {
}
type thisShouldBeString = A extends { bla?: infer T } ? T : never;
It's worth mentioning that when using an `implements` clause, it doesn't actually alter the type of the class where it's applied. It serves as a validation mechanism to ensure the class conforms to the implemented type, but beyond this check, everything functions as if the clause is not present. Therefore, in terms of type inference,
class A implements B<string> { }
is essentially equivalent to
class A { }
This signifies that A essentially acts as an empty class, leading to unexpected behaviors.
Specifically, when attempting to utilize conditional type inference to deduce a value for T under the condition A extends B<T>, it may not yield the desired outcome. If a subtype of B<T> does not include the property bla, then T remains unused, resulting in failed inference. In essence, A is considered assignable to B<T> for any possible T. Consequently, the compiler lacks grounds to favor one type over another, causing the inference process to default to the unknown type:
type ThisShouldBeString = A extends B<infer T> ? T : never;
//type ThisShouldBeString = unknown 😢
To potentially resolve this issue, establishing a structural dependency on T within the comparison between A and
B<T></code becomes crucial. For instance:</p>
<pre><code>class A /* implements B<string> */ {
declare bla?: string;
}
In this scenario, A possesses a structure against which it can be meaningfully contrasted with
B<T></code, even without the explicit <code>implementstype ThisShouldBeString = A extends B<infer T> ? T : never;
//type ThisShouldBeString = string 😃
Celebrate!
Incidentally, this intricacy is independent of the generated JavaScript version of A. Given the presence of a declared bla property, it will not manifest at runtime. Both variants of A transform into something akin to class A {} during compilation. The distinction lies solely within the realm of static types.
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