function foo(a: number, b: number) {
return a + b;
}
type Foo1 = typeof foo extends (...args: unknown[]) => unknown ? true : false; // false
type Foo2 = typeof foo extends (...args: any[]) => unknown ? true : false; // true
What is the reason it functions correctly with any[], but not with unknown[]?
The behavior is successful with tuples and ReturnTypes, but not with rest parameters.
type Foo = ['bar', 'baz'] extends unknown[] ? true : false; // true
Update: Please refer to the comment on the original post for clarification, as the following response was provided based on inaccurate information I previously had about Typescript.
The reason for this issue is due to the constraints imposed by assigning a value to an unknown type. For instance, variables of the number data type can receive values from variables with the any type, but are unable to accept values from variables with the unknown type.
For instance:
let value:any = 5;
let value2:unknown = 6;
let b:number = value;
let c:number = value2;
In this scenario, variable b will compile successfully, while variable c will not, resulting in the error message "Type 'unknown' is not assignable to type 'number'."
Comparing functions in TypeScript can be unexpectedly complex and unintuitive, as highlighted in the specific section of the handbook addressing this topic.
The code example presented reveals a challenge related to function parameter bivariance. A workaround involves comparing the parameters and return type of one function with those of another function independently. Take a look at the following:
In response to your query, Tobias S. referenced another explanation elaborating on various types of variance.
type Fn<Params extends readonly any[] = readonly any[], Result = any> =
(...params: Params) => Result;
type FnAExtendsFnB<FnA extends Fn, FnB extends Fn> =
Parameters<FnA> extends Parameters<FnB>
? ReturnType<FnA> extends ReturnType<FnB>
? true
: false
: false;
type AExtendsB<A, B> = A extends B ? true : false;
// The function from your question:
type Foo = Fn<[a: number, b: number], number>;
type UnknownParamsUnknownResult = Fn<unknown[], unknown>;
type AnyParamsUnknownResult = Fn<any[], unknown>;
declare const T1: AExtendsB<Foo, UnknownParamsUnknownResult>; // false
declare const T2: AExtendsB<Foo, AnyParamsUnknownResult>; // true
declare const T3: FnAExtendsFnB<Foo, UnknownParamsUnknownResult>; // true
declare const T4: FnAExtendsFnB<Foo, UnknownParamsUnknownResult>; // true
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