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// conditionally promisify a function.
// only return a promise if a callback is omitted
export default function awaitify (asyncFn, arity = asyncFn.length) {
if (!arity) throw new Error('arity is undefined')
function awaitable (...args) {
if (typeof args[arity - 1] === 'function') {
return asyncFn.apply(this, args)
}
return new Promise((resolve, reject) => {
args[arity - 1] = (err, ...cbArgs) => {
if (err) return reject(err)
resolve(cbArgs.length > 1 ? cbArgs : cbArgs[0])
}
asyncFn.apply(this, args)
})
}
return awaitable
}
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