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.
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
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 }])
testing([{ key: "one", value: "one" }, { key: 2, value: "two" }]).
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 }])
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)[]
value could be.
any, no?anytype should be avoidedanyisunknown, it forces the caller to type check it.anyorunknown.