If you need to utilize a generic function or create a generic class with multiple type parameters, the options are either to explicitly define all type arguments or allow the compiler to deduce them. TypeScript currently lacks support for partial inference of type arguments where only some are specified and others are inferred. A request has been made to address this limitation in future versions of the language, but as of now, it remains unchanged.
One approach is to specify the type that you prefer not to infer using the `typeof` type query operator:
const params = { a: '1', b: '2', c: '3 ' };
const foo = new Foo<Value1, typeof params>({ params });
// const foo: Foo<Value1, { a: string; b: string; c: string; }>
Another workaround involves currying the generic function at the type level, which adds an additional function call. For generic classes, a factory function may be required to achieve similar functionality.
You could implement this strategy in your example like so:
class _Foo<T1, T2>{
a: string = 'a'
constructor(private readonly config: { params: T2 }) { }
selectT2(v: keyof T2) { }
selectT1(t: keyof T1) { }
}
function Foo<T1>() {
const Foo: new <T2>(config: { params: T2 }) => Foo<T1, T2> = _Foo;
return Foo;
}
type Foo<T1, T2> = _Foo<T1, T2>;
Yet another workaround involves introducing a dummy parameter corresponding to the manually defined type, allowing the compiler to infer the type from the supplied dummy argument:
class Foo<T1, T2>{
a: string = 'a'
constructor(dummyT1: T1, private readonly config: { params: T2 }) { }
selectT2(v: keyof T2) { }
selectT1(t: keyof T1) { }
}
const foo = new Foo(
null! as Value1, // Asserting the type
{ params: { a: '1', b: '2', c: '3 ' } }
);
// const foo: Foo<Value1, { a: string; b: string; c: string; }>
These workarounds present different ways to handle the lack of partial type argument inference in TypeScript. Each has its own set of trade-offs, and the choice depends on the specific requirements of your use case. Hopefully, future improvements to the language will provide more straightforward solutions for addressing this issue.
Playground link to code