Compiler is unable to comprehend the conditional return type

Question

Compiler is unable to comprehend the conditional return type

I've done some searching, but unfortunately haven't been able to find a definitive solution to my issue. Apologies if this has already been asked before, I would appreciate any guidance you can offer.

The problem I'm facing involves a store class that I created for the current project. Here's an example of a function within it:

public getStore(store: 'FIELD1' | 'FIELD2'): State<typeof store> {
 return this[store];
}

This function is accompanied by a conditional type in the following file:

export type State<T> = T extends 'FIELD1'
  ? Field1Type[]
  : T extends 'FIELD2'
  ? Date
  : never;

I was expecting this setup to resolve the compilation error when trying to access getStore('FIELD1').length, as it should allow for either Field1Type[] or Date as potential returns. However, this isn't happening.

This situation is proving to be inconvenient for me, especially since I'm working on an Angular app and wanted to directly call this function in the component's HTML without relying on additional helpers within the script. Am I missing something here, or is this behavior expected?

angular typescript generics

Answer 1

Answer №1

Your function currently returns a combination of Date | Field1Type[]. This means that it can return either a Date or an array of Field1Type, regardless of the input provided. Therefore, trying to access the length property is not permitted.

To align the return type with the input, you should explore using function overloads:

getStore(store: 'FIELD1'): Field1Type[]
getStore(store: 'FIELD2'): Date
getStore(store: 'FIELD1' | 'FIELD2'): Date | Field1Type[] {
    return this[store];
}

If your implementation relies on a generic type like State, you can incorporate it in the function as well:

getStore<T extends 'FIELD1' | 'FIELD2'>(store: T): State<T>
getStore(store: 'FIELD1' | 'FIELD2'): Date | Field1Type[] {
    return this[store];
}

export type State<T> = T extends 'FIELD1'
  ? Field1Type[]
  : T extends 'FIELD2'
  ? Date
  : never;

Consider simplifying your State implementation by using a map-like structure:

export type State<T extends string> = ({
  FIELD1: Field1Type[]
  FIELD2: Date
} & Record<string, never>)[T]

Answer 2

Your function currently returns a combination of Date | Field1Type[]. This means that it can return either a Date or an array of Field1Type, regardless of the input provided. Therefore, trying to access the length property is not permitted.

To align the return type with the input, you should explore using function overloads:

getStore(store: 'FIELD1'): Field1Type[]
getStore(store: 'FIELD2'): Date
getStore(store: 'FIELD1' | 'FIELD2'): Date | Field1Type[] {
    return this[store];
}

If your implementation relies on a generic type like State, you can incorporate it in the function as well:

getStore<T extends 'FIELD1' | 'FIELD2'>(store: T): State<T>
getStore(store: 'FIELD1' | 'FIELD2'): Date | Field1Type[] {
    return this[store];
}

export type State<T> = T extends 'FIELD1'
  ? Field1Type[]
  : T extends 'FIELD2'
  ? Date
  : never;

Consider simplifying your State implementation by using a map-like structure:

export type State<T extends string> = ({
  FIELD1: Field1Type[]
  FIELD2: Date
} & Record<string, never>)[T]

Compiler is unable to comprehend the conditional return type

Answer №1

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