[英]Typescript conditional generic type in return type
如果值不存在,如何调节返回类型的泛型?
type Foo = {};
class Bar<P extends Foo> {
static make<P extends Foo>(a?: P): Bar<P> {
return new Bar();
}
}
Bar.make() // return Bar<Foo>
但我需要做类似的事情:
class Bar<P extends Foo> {
static make<P extends Foo>(a?: P): Bar<P extends undefined ? never : Foo> {
return new Bar();
}
}
Bar.make() // return Bar<never>
Bar.make({}) // return Bar<Foo>
您需要应用默认类型never
。
type Foo = {test: number}; // Example implementation of foo, used in test cases below
class Bar<P extends Foo> {
// Allow P to extend from Foo, then assign never as the default
static make<P extends Foo = never>(a?: P): Bar<P> {
return new Bar();
}
}
const t1 = Bar.make(); // Bar<never>
const t2 = Bar.make({ test: 2 }) // Bar<{test: 2}> (subtype of Foo)
const t3: Bar<Foo> = Bar.make({ test: 2 }); // Bar<Foo> (explicit typecasting to Bar<Foo>)
const t4 = Bar.make({ bazz: 0 }); // Type error, { bazz: 0 } is not assignable to Foo
这里有一个TypeScript Playground 链接来展示不同的结果。
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