Generic array type in Typescript-CodePudding

I have function with generic type.

But it return an error Type 'boolean' is not assignable to type 'string'.

function testing<K, V>(array: Array<{ key: K, value: V }>): Array<V> {
    return array.map(data => data.value)
}

testing([{ key: 1, value: "one" }, { key: 2, value: true }])

How can I fix this generic type issue with keeping the type generic. Because the array is unknown and the value may have different custom type.

CodePudding user response：

TS apparently isn't smart enough to infer V being an union type, so for tricky arguments like that you'll have to declare it explicitly.

function testing<K, V>(array: Array<{ key: K, value: V }>): Array<V> {
    return array.map(data => data.value)
}

testing<number, string | boolean>(
    [{ key: 1, value: "one" }, { key: 2, value: true }]
); // works, TS is told explicitly what V is

testing(
    [{ key: 1, value: "one" }, { key: 2, value: "two" }]
); // works, TS correctly infers string

CodePudding user response：

You don't really know the type of value is. You should use unknown to force users to type check it.

I also don't see why you need key and K since they are ignored.

function testing(array: Array<{value: unknown }>): Array<unknown> {
    return array.map(data => data.value);
}

testing([{ key: 1, value: "one" }, { key: 2, value: true }])

CodePudding user response：

This should work:

function testing<K, V>(array: Array<{ key: K, value: V }>): Array<V> {
    return array.map(data => data.value)
}

testing<number, string | boolean>([{ key: 1, value: "one" }, { key: 2, value: true }])

CodePudding user response：

The following will extract the type of each value and return the union:

type ArrayValueTypes<MyValueArray extends { value: unknown }[]> = {
    [K in keyof MyValueArray]: MyValueArray[K]['value']
}

const myArray = [{ key: 1, value: "one" }, { key: 2, value: true }]

function testing<T extends { value: any }[]>(array: T): ArrayValueTypes<T> {
    return array.map(data => data.value)
}

const result = testing(myArray)
// result is of type (string | boolean)[]

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