How can I condition the generic return type when a value is not present?
type Foo = {};
class Bar<P extends Foo> {
static make<P extends Foo>(a?: P): Bar<P> {
return new Bar();
}
}
Bar.make() // returns Bar<Foo>
However, I would like to achieve something like this:
class Bar<P extends Foo> {
static make<P extends Foo>(a?: P): Bar<P extends undefined ? never : Foo> {
return new Bar();
}
}
Bar.make() // returns Bar<never>
Bar.make({}) // returns Bar<Foo>
To ensure proper functionality, it is important to set the default type as never.
type Foo = {test: number}; // An example implementation of foo used in test cases below
class Bar<P extends Foo> {
// By allowing P to extend from Foo and setting never as the default
static make<P extends Foo = never>(a?: P): Bar<P> {
return new Bar();
}
}
const t1 = Bar.make(); // Results in Bar<never>
const t2 = Bar.make({ test: 2 }) // Generates Bar<{test: 2}> (subtype of Foo)
const t3: Bar<Foo> = Bar.make({ test: 2 }); // Outputs Bar<Foo> (explicit casting to Bar<Foo>)
const t4 = Bar.make({ bazz: 0 }); // Error in typing, { bazz: 0 } cannot be assigned to Foo
Follow this TypeScript Playground link for a demonstration of the various scenarios.
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