/
index.js
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/
index.js
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import AggregateError from 'aggregate-error';
export default async function pMap(
iterable,
mapper,
{
concurrency = Number.POSITIVE_INFINITY,
stopOnError = true
} = {}
) {
return new Promise((resolve, reject) => {
if (typeof mapper !== 'function') {
throw new TypeError('Mapper function is required');
}
if (!((Number.isSafeInteger(concurrency) || concurrency === Number.POSITIVE_INFINITY) && concurrency >= 1)) {
throw new TypeError(`Expected \`concurrency\` to be an integer from 1 and up or \`Infinity\`, got \`${concurrency}\` (${typeof concurrency})`);
}
const result = [];
const errors = [];
const skippedIndexes = [];
let isRejectedOrResolved = false;
let isIterableDone = false;
let resolvingCount = 0;
let currentIndex = 0;
const iterator = iterable[Symbol.iterator] === undefined ? iterable[Symbol.asyncIterator]() : iterable[Symbol.iterator]();
const next = async () => {
if (isRejectedOrResolved) {
return;
}
const nextItem = await iterator.next();
const index = currentIndex;
currentIndex++;
// Note: iterator.next() can be called many times in parallel.
// This can cause multiple calls to this next() function to
// receive a `nextItem` with `done === true`.
// The shutdown logic that rejects/resolves must be protected
// so it runs only one time as the `skippedIndex` logic is
// non-idempotent.
if (nextItem.done) {
isIterableDone = true;
if (resolvingCount === 0 && !isRejectedOrResolved) {
isRejectedOrResolved = true;
if (!stopOnError && errors.length > 0) {
reject(new AggregateError(errors));
} else {
for (const skippedIndex of skippedIndexes) {
result.splice(skippedIndex, 1);
}
resolve(result);
}
}
return;
}
resolvingCount++;
// Intentionally detached
(async () => {
try {
const element = await nextItem.value;
if (isRejectedOrResolved) {
return;
}
const value = await mapper(element, index);
if (value === pMapSkip) {
skippedIndexes.push(index);
} else {
result[index] = value;
}
resolvingCount--;
await next();
} catch (error) {
if (stopOnError) {
isRejectedOrResolved = true;
reject(error);
} else {
errors.push(error);
resolvingCount--;
await next();
}
}
})();
};
// Create the concurrent runners in a detached (non-awaited)
// promise. We need this so we can await the next() calls
// to stop creating runners before hitting the concurrency limit
// if the iterable has already been marked as done.
// NOTE: We *must* do this for async iterators otherwise we'll spin up
// infinite next() calls by default and never start the event loop.
(async () => {
for (let index = 0; index < concurrency; index++) {
try {
// eslint-disable-next-line no-await-in-loop
await next();
} catch {
break;
}
if (isIterableDone) {
break;
}
}
})();
});
}
export const pMapSkip = Symbol('skip');