63 lines
1.3 KiB
TypeScript
63 lines
1.3 KiB
TypeScript
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/**
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* @packageDocumentation
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*
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* Return the first value in an (async)iterable
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*
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* @example
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*
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* ```javascript
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* import first from 'it-first'
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*
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* // This can also be an iterator, generator, etc
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* const values = [0, 1, 2, 3, 4]
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*
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* const res = first(values)
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*
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* console.info(res) // 0
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* ```
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*
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* Async sources must be awaited:
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*
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* ```javascript
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* import first from 'it-first'
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*
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* const values = async function * () {
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* yield * [0, 1, 2, 3, 4]
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* }
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*
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* const res = await first(values())
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*
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* console.info(res) // 0
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* ```
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*/
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function isAsyncIterable <T> (thing: any): thing is AsyncIterable<T> {
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return thing[Symbol.asyncIterator] != null
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}
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/**
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* Returns the first result from an (async) iterable, unless empty, in which
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* case returns `undefined`
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*/
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function first <T> (source: Iterable<T>): T | undefined
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function first <T> (source: Iterable<T> | AsyncIterable<T>): Promise<T | undefined>
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function first <T> (source: Iterable<T> | AsyncIterable<T>): Promise<T | undefined> | T | undefined {
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if (isAsyncIterable(source)) {
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return (async () => {
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for await (const entry of source) { // eslint-disable-line no-unreachable-loop
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return entry
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}
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return undefined
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})()
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}
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for (const entry of source) { // eslint-disable-line no-unreachable-loop
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return entry
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}
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return undefined
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}
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export default first
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