How To Use Generics Properly To Infer Return Type of Function Correctly?
I've the following types
type ItemDefaultType = object | null | string
interface ItemToString<Item = ItemDefaultType> {
(item: ItemDefaultType): string;
}
interface AutosuggestState<Item = ItemDefaultType> {
highlightedIndex: number | null
inputValue: string | null
isOpen: boolean
selectedItem: Item
}
interface AutosuggestProps<Item = ItemDefaultType> extends AutosuggestState<Item> {
itemToString?: ItemToString<Item>;
initialSelectedItem?: Item;
initialInputValue?: string;
initialHighlightedIndex?: number | null;
initialIsOpen?: boolean;
defaultHighlightedIndex?: number | null;
defaultIsOpen?: boolean;
id?: string;
inputId?: string;
labelId?: string;
menuId?: string;
itemCount?: number
}
I want to make a getInitialValue
function with the intended signature β
// this has to be done correctly using generics, not been able to do it, read onβ¦
(
props: Props extends AutosuggestProps,
stateKey: StateKey extends keyof AutosuggestState
) => AutosuggestState[StateKey] // return the correct type of AutosuggestState property based on which stateKey was passed
The behaviour of getInitialValue
looks like this
const defaultStateValues = {
highlightedIndex: -1,
isOpen: false,
selectedItem: null,
inputValue: ''
}
function getDefaultValue(props, statePropKey) {
const defaultPropKey = `default${capitalizeString(statePropKey)}`
if (defaultPropKey in props) {
return props[defaultPropKey]
}
return defaultStateValues[statePropKey]
}
function getInitialValue(props, statePropKey) {
if (statePropKey in props) {
return props[statePropKey]
}
const initialPropKey = `initial${capitalizeString(statePropKey)}`
if (initialPropKey in props) {
return props[initialPropKey]
}
return getDefaultValue(props, statePropKey)
}
I'm finding it hard to write types of both getInitialValue
and getDefaultValue
such that getInitialValue correctly infers the right type as below β
const selectedItem = getInitialValue(props, 'selectedItem') // selectedItem variable should correctly be inferred as **object | null | string** since that's what its type is in **AutosuggestState** interface
Can someone help me write the types?