Ytukyg

When I run this code:

type Converter<T extends ConverterOutput> = (value: string) => T

type ConverterOutput = number | string



function generateConverter<T extends ConverterOutput>(

  value: string,

  type: 'number' | 'string'

): Converter<T> {

  return (value: string): T => {

    switch (type) {

      case 'number':

        return Number(value)



      default:

        return value

    }

  }

}

I get these 2 errors:

• [ts] Type 'number' is not assignable to type 'T'. [2322]


• [ts] Type 'string' is not assignable to type 'T'. [2322]

for the 2 switch ... case returned values. I don't get what can be wrong in this code since T extends ConverterOutput which itself is a number | string.

I have tried adding a generic catch: return <U extends T>(value: string): U => { but it doesn't solve anything other than telling me that it's not assignable to U this time.

Typescript will not let you assign concrete values where a value of a generic type parameter is expected. The reason for this is that usually the value can't be validated against all possible derived types that could conceivable be passed for the generic type parameter. For example even in your case, T extends number| string so this mean it could be the number literal type 1 which would not be satisfied by the value Number(value).

In your case I would recommend doing away with the generic type parameters and use multiple overloads, with the implementation signature returning Converter<number | string>. This has the added benefit that the type of the converter will only need to be specified as a string not as both string and type parameter.

type Converter<T extends ConverterOutput> = (value: string) => T

type ConverterOutput = number | string





function generateConverter(value: string, type: 'number'): Converter<number> 

function generateConverter(value: string, type: 'string'): Converter<string> 

function generateConverter(value: string, type: 'number' | 'string'): Converter<number | string> {

  return (value) => {

    switch (type) {

      case 'number':

        return Number(value)



      default:

        return value

    }

  }

}

Because a number nor a string does not extend itself. That's why it's not working. You can fix it by giving the type directly e.g.

function generateConverter(value: string, type: 'number' | 'string'): Converter<ConverterOutput> {

    return (value: string): ConverterOutput => {

        switch (type) {

            case 'number':

                return Number(value)



            default:

                return value

        }

    }

}