Determine the callback parameter type by analyzing the function parameter

type SeveralTypes = type0 | type1 | type2;
function funTypes<T extends SeveralTypes>(info: T, callback: (obj: T extends type2 ? number : string) => void) {
    if (isType0(info)) {
        return callback("passAstring");  // TS Warn: Argument of type '"passAstring"' is not assignable to parameter of type 'T extends boolean ? number : string'

    } else if (isType1(info)) {
        return callback("passAstring"); // TS Warn: Argument of type '"passAstring"' is not assignable to parameter of type 'T extends boolean ? number : string'

    } else {
        return callback(1001); // TS Warn: Argument of type '1001' is not assignable to parameter of type 'T extends boolean ? number : string'
    }
}

funTypes(1, (d) => { });       // Typeof d --> string
funTypes("str", (d) => { });   // Typeof d --> string
funTypes(false, (d) => { });   // Typeof d --> number

While using this function, the inferred type of the parameter for the callback seems correct. However, TypeScript flags issues with assigning parameters. Is there an alternative method to type the callback parameters?

typescripttype-inference

Answer №1

It is important to address various issues in this code snippet. One of the main concerns is that narrowing one variable (info) does not affect the type of another variable (callback). Additionally, conditional types containing unresolved type parameters can be challenging for the compiler to interpret. Since the type T is unknown, TypeScript plays it safe by not allowing the use of either number or string as a parameter.

To work around this issue, one common approach is to employ type assertions or utilize a combination of a generic public signature and a more permissive implementation signature. However, it's essential to ensure that the logic within the conditional type is replicated accurately in the implementation.

type SeveralTypes = type0 | type1 | type2;
function funTypes<T extends SeveralTypes>(info: T, callback: (obj: T extends type2 ? number : string) => void): void
function funTypes(info: SeveralTypes, callback: (obj: number | string) => void) {
  if (isType0(info)) {
    return callback("passAstring");

  } else if (isType1(info)) {
    return callback("passAstring");

  } else {
    return callback(1001); 
  }
}

funTypes(1, (d) => { });       // Typeof d --> string
funTypes("str", (d) => { });   // Typeof d --> string
funTypes(false, (d) => { });   // Typeof d --> number

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