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export type LazyResult<TValue> = PromiseLike<TValue> & { value?: TValue }
export type ResolvedLazyResult<TValue> = PromiseLike<TValue> & { value: TValue }
/**
* Calls the given async function only when the returned promise-like object is
* awaited. Afterwards, it provides the resolved value synchronously as `value`
* property.
*/
export function createLazyResult<TValue>(
fn: () => Promise<TValue>
): LazyResult<TValue> {
let pendingResult: Promise<TValue> | undefined
const result: LazyResult<TValue> = {
then(onfulfilled, onrejected) {
if (!pendingResult) {
pendingResult = fn()
}
pendingResult
.then((value) => {
result.value = value
})
.catch(() => {
// The externally awaited result will be rejected via `onrejected`. We
// don't need to handle it here. But we do want to avoid an unhandled
// rejection.
})
return pendingResult.then(onfulfilled, onrejected)
},
}
return result
}
export function isResolvedLazyResult<TValue>(
result: LazyResult<TValue>
): result is ResolvedLazyResult<TValue> {
return result.hasOwnProperty('value')
}
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