Data types for keys and values in TypeScript

Question

Data types for keys and values in TypeScript

interface A { a?: number };
interface B { a?: string };

function replicate<
    Source extends object,
    Destination extends { [key in keyof Source]?: (1) }
>(
    source: Source,
    key: keyof Source,
    destination: Destination,
    converter: (value: (2)) => (3) 
) {
    if (source[key] !== undefined) {
        destination[key] = converter ? converter(source[key]) : source[key];
    }
}

const a: A = { a: 123 };
const b: B = {};

replicate(a, "a", b, (value) => value.toString());

In the sample above, what fits for the following placeholders:

(1) - data type of value in Destination related to the corresponding key in Source.
(2) - information type in Source associated with the specified key in parameter key.
(3) - data type in Destination linked to the specified key in parameter key.

typescript generics types typescript3.0

Answer 1

Answer №1

To ensure accurate key passing, an additional type parameter is needed to represent the specific key being passed in. This parameter will be automatically determined as a string literal type based on the value passed into the key parameter. By introducing this new type, we can utilize type queries to access the exact property within the Source and Destination types.

In addition, since our main focus lies on the particular K key, we can incorporate it while defining the Destination type by specifying its presence (instead of mandating that Destination must contain all keys from Source). As we are not concerned with the destination property's type, but rather with its existence and the fact that the transformer function should return a value of the same type as this property, we can simply designate the type of the property in Destination as unknown.

interface A { a?: number };
interface B { a?: string };

function copy<
    K extends keyof Source, // Additional type parameter
    Source extends object,
    Destination extends { [destinationKey in K]?: unknown } // Can be any type without much concern, solely the existence of key K matters
>(
    source: Source,
    key: K, // Key is of type K
    destination: Destination,
    transformer: (value: Source[K]) => Destination[K] // Utilizing K to define the relationship between Source and Destination property types
) {
    if (source[key] !== undefined) {
        destination[key] = transformer ? transformer(source[key]) : source[key];
    }
}

const a: A = { a: 123 };
const b: B = {};

copy(a, "a", b, (value) => value.toString());
copy(a, "a", b, (value) => value); /// error

Answer 2

To ensure accurate key passing, an additional type parameter is needed to represent the specific key being passed in. This parameter will be automatically determined as a string literal type based on the value passed into the key parameter. By introducing this new type, we can utilize type queries to access the exact property within the Source and Destination types.

In addition, since our main focus lies on the particular K key, we can incorporate it while defining the Destination type by specifying its presence (instead of mandating that Destination must contain all keys from Source). As we are not concerned with the destination property's type, but rather with its existence and the fact that the transformer function should return a value of the same type as this property, we can simply designate the type of the property in Destination as unknown.

interface A { a?: number };
interface B { a?: string };

function copy<
    K extends keyof Source, // Additional type parameter
    Source extends object,
    Destination extends { [destinationKey in K]?: unknown } // Can be any type without much concern, solely the existence of key K matters
>(
    source: Source,
    key: K, // Key is of type K
    destination: Destination,
    transformer: (value: Source[K]) => Destination[K] // Utilizing K to define the relationship between Source and Destination property types
) {
    if (source[key] !== undefined) {
        destination[key] = transformer ? transformer(source[key]) : source[key];
    }
}

const a: A = { a: 123 };
const b: B = {};

copy(a, "a", b, (value) => value.toString());
copy(a, "a", b, (value) => value); /// error

Data types for keys and values in TypeScript

Answer №1

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