How can I access the keys of an interface in which the type of interface[key] is defined?

Question

How can I access the keys of an interface in which the type of interface[key] is defined?

Consider this scenario:

interface Interface {
  a: number
  b: number
  c: string
  d: string
  e: number[]
}

If I want to retrieve the keys of Interface based on the type of their corresponding values in Interface, how can I achieve that? This is similar to the functionality of Pick<T, K>, but instead of selecting keys based on their type, we are interested in keys whose values match a certain type.

An example of what I am trying to accomplish is shown below:

type KeyOfWhereValueMatches<T, U extends U[keyof U]> = // ???
KeyOfWhereValueMatches<Interface, number> // 'a' | 'b'
KeyOfWhereValueMatches<Interface, string> // 'c' | 'd'
KeyOfWhereValueMatches<Interface, number[]> // 'e'

typescript types

Answer 1

Answer №1

To achieve this functionality, you can use the following code snippet:

type GetKeysWhereValueMatches<
    Input,
    Value,
    Result = { [property in keyof Input]: Value extends Input[property] ? property : never; }
    > = Result[keyof Result]

Here's how it operates: Input represents your interface, while Value signifies the type for which you need to retrieve keys.

The variable Result is a type that iterates over each key within the interface type. If the assigned value matches the type of that key (e.g. string extends Interface[k]), the resulting type for that field becomes a string literal of its key. Otherwise, it defaults to a never.

For instance, invoking

GetKeysWhereValueMatches<Interface, string>

would yield:

interface Result {
    name: "name";
    age: never;
    gender: "gender";
}

Lastly, Result[keyof Result] utilizes lookup types to ascertain the types corresponding to all available keys in Result. These could either be the appropriate key's string literals or never. Given that never isn't encountered, the lookup simply returns the string literals.

Answer 2

To achieve this functionality, you can use the following code snippet:

type GetKeysWhereValueMatches<
    Input,
    Value,
    Result = { [property in keyof Input]: Value extends Input[property] ? property : never; }
    > = Result[keyof Result]

Here's how it operates: Input represents your interface, while Value signifies the type for which you need to retrieve keys.

The variable Result is a type that iterates over each key within the interface type. If the assigned value matches the type of that key (e.g. string extends Interface[k]), the resulting type for that field becomes a string literal of its key. Otherwise, it defaults to a never.

For instance, invoking

GetKeysWhereValueMatches<Interface, string>

would yield:

interface Result {
    name: "name";
    age: never;
    gender: "gender";
}

Lastly, Result[keyof Result] utilizes lookup types to ascertain the types corresponding to all available keys in Result. These could either be the appropriate key's string literals or never. Given that never isn't encountered, the lookup simply returns the string literals.

How can I access the keys of an interface in which the type of interface[key] is defined?

Answer №1

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