Perfecting compound types

Having trouble with this code snippet:

type FormatTypes = {
    text: string,
    binary: Array<number>
};

type Format = keyof FormatTypes;
type FormatType<F extends Format> = FormatTypes[F];

type Formatter = {
    format<F extends Format>(param: string, format: F): FormatType<F>
}

const blah: Formatter = {
    format(param, format) {
        return format === 'text' ? param : [1, 0, 0, 1];
    }
};

Why am I getting this error message?

Type '<F extends "text" | "binary">(param: string, format: F) => string | number[]' is not assignable to type '<F extends "text" | "binary">(param: string, format: F) => FormatType<F>'.
  Type 'string | number[]' is not assignable to type 'FormatType<F>'.
    Type 'string' is not assignable to type 'FormatType<F>'.
      Type 'string' is not assignable to type 'string & number[]'.
        Type 'string' is not assignable to type 'number[]'.(2322)
input.tsx(10, 5): The expected type comes from property 'format' which is declared here on type 'Formatter'

Any ideas why the conditional statement is not properly refining the type of the format parameter for the return value? Any suggestions on how to achieve the desired outcome?

typescriptunion-types

Answer №1

Unlike the keyof parameter, the FormatType<F> does not always result in string | Array<number> in this scenario.

I successfully compiled the program by modifying your final blah object as shown below.

I could not find a more concise way to handle this situation.

// option 1: utilize the `format` parameter
const blah: Formatter = {
  format: (param, format) => {
    return (format === "text" ? param : [1, 0, 0, 1]) as FormatType<typeof format>
  },
};

//option 2: reintroduce generics here
const blah: Formatter = {
  format: <F extends Format>(param:string, format:F) => {
    return (format === "text" ? param : [1, 0, 0, 1]) as FormatType<F>
  },
};

Answer №2

When defining FormatType, you are utilizing the concept of indexed access operator as described in this resource:

type FormatType<F extends Format> = FormatTypes[F];

It's important to note that this method only applies to actual indexed access, meaning that the compiler can validate typings when accessing object properties but may not work effectively in all control structures due to limitations in the compiler's ability to track types accurately throughout the entire program.

To achieve your desired outcome, consider revising your code to implement genuine indexed access, for example:

const blah: Formatter = {
    format(param, format) {
        const formatTypes = {
            text: param,
            binary: [1, 0, 0, 1]
        }
        return formatTypes[format]
    }
};

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