I wrote three functions with same type for each of them, let's see them.

const func: <T extends string, K extends number = any>(arg: T) => K = () => {
  return 1;
};

const anotherFunc = <T extends string, K extends number = any>(arg: T): K => {
  return 1;
};

function someFunc<T extends string, K extends number = any>(arg: T): K {
  return 1;
}

as you can see i defined K as number in all of them to returns a number in each of them, but a weird error occured that says:

this is for the first function:

Type '() => 1' is not assignable to type '<T extends string, K extends number = any>(arg: T) => K'.
  Type 'number' is not assignable to type 'K'.
    'number' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'number'.

and this is for the last two functions:

Type 'number' is not assignable to type 'K'.
  'number' is assignable to the constraint of type 'K', but 'K' could be instantiated with a different subtype of constraint 'number'

i think the two errors have same point. why typescript can't realize that type?

thanks for any help.

CodePudding user response：

I'm not sure whether this is a duplicate or not, but please take a look at this answer

Here is the simplified version of first example:

const func: <K extends number>() => K = () => {
    return 1;
};

Now, try to call this function:const result = func<42>(); // 42. TS assumes, return type/value is 42, but it is 1 in runtime.

Further more, 42 & { WAAAT: () => '?????' } is a valid subtype of number.

const result = func<42 & { WAAAT: () => '?????' }>();
result.WAAAT() // ?????

Above code compiles in TypeScript but throws a runtime error in runtime. Please see the error message:

K could be instantiated with a different subtype of constraint number

42 & { WAAAT: () => '?????' } is a valid subtype of constraint number.

Hence, don't treat extends as equal operator.

Please see my article for more examples

CodePudding user response：

Couple of observations.

• why do u have args:T if you never use it?
• why are you extending a primitive string and number and assigning an any type to K?
• your first function is wrong. Lets transformer to a classic function

const func: <T extends string, K extends number = any> function (args:T) {
  return K = function () { 
return 1;
}
}

Did you see the two errors ?

• to assign value we use = which you are not using in func look at anotherFunc for example
• you forgot to add the third error which Is

Declaration or statement expected.