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ryangoree/DefinedWhen.ts

Last active September 14, 2023 21:55
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Experiment for dynamic interfaces
Raw
DefinedWhen.ts
/**
* Construct a type based off T where TDefinedField is defined when
* T matches TDefinedState.
*/
type DefinedWhen<
T extends Record<string, any>,
TDefinedField extends keyof T,
TDefinedState extends Partial<T>,
> =
| (Omit<T, TDefinedField | keyof TDefinedState> &
TDefinedState &
Record<TDefinedField, Exclude<T[TDefinedField], undefined>>)
| {
[DefinedStateKey in keyof TDefinedState]: {
[K in keyof T]: K extends DefinedStateKey
? Exclude<T[K], TDefinedState[K]>
: T[K];
};
}[keyof TDefinedState];
// =======================================================================================
// Example:
// ---------------------------------------------------------------------------------------
type Payload = DefinedWhen<
{
isFetched: boolean;
hasError: boolean;
data: string | undefined;
dataType: string | undefined;
meta: string | undefined;
},
"data" | "dataType" | "meta",
{ isFetched: true; hasError: false }
>;
function handleResponse({
isFetched,
hasError,
data,
dataType,
meta,
}: Payload) {
data; // string | undefined
// ^?
dataType; // string | undefined
// ^?
meta; // string | undefined
// ^?
if (isFetched === true && hasError === false) {
data; // string
// ^?
dataType; // string
// ^?
meta; // string
// ^?
}
}
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