Make sense of the error message "Type 'unknown' is not assignable to type 'strin-CodePudding

Here is my isolated code:

type IDontCare = any;

let aFunction: (params: unknown) => IDontCare;
aFunction = (hello: string) => 0;

And I got the error message Type 'unknown' is not assignable to type 'string'. I wonder, isn't it I'm trying to assign 'string' to 'unknown' instead? I can't wrap my head around this error message, and would love to hear some explanation. Thanks.

Code	Error message

CodePudding user response：

When you say (params: A) => number is assignable to (params: B) => number, you expect (params: A) => number behaves the same as (params: B) => number,

i.e. (params: A) => number can take a params of type B and return a number.

That means you are trying to check if a B can be passed into (params: A) => number. The result is you are trying to assign B to A. If B is not assignable to A, then (params: A) => number is not assignable to (params: B) => number.

In other words, you need B to be assignable to A in order for (params: A) => number to be assignable to (params: B) => number

CodePudding user response：

Type of aFunction says, that it can receive argument of any type. So, you should assign to aFunction function that can do the same or even more.

(hello: string) => 0 cannot take any argument (actually, it takes only string), so it is not assignable to aFunction