how to explain to typescript that variable is always a string in generic class-CodePudding

The
Since this picture clear only allows the two keys that are guaranteed to be strings.

How do i make tsc understand this also?
If anything is unclear please let me know so i can modify the question.

CodePudding user response：

It seems the issue is a limitation of Typescript inference capabilities with respect to generic types - Typescript does not keep track all inferable constraints. Definitely there are some tradeoffs in doing so.

For a concrete type the inference works as expected:

const props: SearchListProps<DataExample> = { data: example_data, sort_and_filter_key: 'city', }; const ok_string = props.data[0][props.sort_and_filter_key];

In case of a generic type we need to manually give a hint to TS engine what exactly we are interested in:

interface SearchListProps<T> { data: (T & Record<KeysMatching<T, string>, string>)[]; sort_and_filter_key: KeysMatching<T, string>; }

Interestingly we can intersect T with anything else - for example Record<KeysMatching<T, string>, number>, but then we are not able to initialise SearchListProps with a concrete type.

A fixed and simplified version: https://stackblitz.com/edit/react-ts-x4jdez

Btw. props.data[0][props.sort_and_filter_key] is unsafe (noUncheckedIndexedAccess].

CodePudding user response：

We could also give ItemDataT the constraint Record<K, string> where K is just constrained as string.

class SearchList< ItemDataT extends Record<K, string>, K extends string > extends Component<SearchListProps<ItemDataT, K>> { constructor(props: { data: ItemDataT[], sort_and_filter_key: K }) { /* ... */ } }

When we now access props.data[0][props.sort_and_filter_key], TypeScript still displays the type as ItemDataT[K]. But TypeScript actually knows that this must be a string this time. All the string methods are available. The type is only displayed "wrong" but behaves correctly.

const value = props.data[0][props.sort_and_filter_key]; value.charAt(0) // this was compile time error before

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