The implementation is throwing a value type mismatch error. How can I resolve this issue?

enum A {
    AA = 'AA',
    BB = 'BB'
}

export interface OptionsA {a: number}
export interface OptionsB {b: string}

export interface ValuesA {a: boolean}
export interface ValuesB {b: boolean | null}

export interface FirstMapA {
    [A.AA]: OptionsA,
    [A.BB]: OptionsB
}

export interface SecondMapA {
    [A.AA]: ValuesA,
    [A.BB]: ValuesB
}

export class SomeThing<Type extends keyof FirstMapA> {
    type: Type;
    value: SecondMapA[Type];

    doThings(): void {
        if (this.type === A.AA) {
            this.value.
        }
    }
}

While working on the "doThings" function in this code snippet, there is an issue where "this.value" is defined as ValuesA | ValuesB but it should only be ValuesA. Can you help identify the problem?

typescript

Answer №1

It seems that TypeScript struggles to recognize that this.type === A.AA specifically limits the type of this.value to ValuesA. Instead, TypeScript maintains the possibility that this.value could be either ValuesA or ValuesB.

To clarify this for TypeScript, you can utilize a type assertion to indicate that this.value should be considered as ValuesA when this.type is set to A.AA.

enum A {
  AA = "AA",
  BB = "BB",
}

export interface OptionsA {
  a: number;
}
export interface OptionsB {
  b: string;
}

export interface ValuesA {
  a: boolean;
}
export interface ValuesB {
  b: boolean | null;
}

export interface FirstMapA {
  [A.AA]: OptionsA;
  [A.BB]: OptionsB;
}

export interface SecondMapA {
  [A.AA]: ValuesA;
  [A.BB]: ValuesB;
}

export class SomeThing<Type extends keyof FirstMapA> {
  type: Type;
  value: SecondMapA[Type];

  doThings(): void {
    if (this.type === A.AA) {
      const valueA = this.value as ValuesA; // Type assertion
      valueA.a; // TypeScript now recognizes this as ValuesA
    }
  }
}

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