|
| 1 | +```js |
| 2 | +/** |
| 3 | + * Implements Promise.all functionality. |
| 4 | + * |
| 5 | + * @param {Iterable} iterable - An array (or any iterable) of promises or values. |
| 6 | + * @returns {Promise} - A promise that resolves when all input promises resolve, |
| 7 | + * or rejects when any input promise rejects. |
| 8 | + */ |
| 9 | +export default function promiseAll(iterable) { |
| 10 | + if (!iterable || typeof iterable[Symbol.iterator] !== 'function') { |
| 11 | + // Ensure the input is an iterable |
| 12 | + throw new TypeError('Expected an iterable'); |
| 13 | + } |
| 14 | + |
| 15 | + return new Promise((resolve, reject) => { |
| 16 | + const results = []; // Array to store resolved values |
| 17 | + let completed = 0; // Counter for how many promises have been resolved |
| 18 | + const promises = Array.from(iterable); // Convert iterable to an array |
| 19 | + |
| 20 | + if (promises.length === 0) { |
| 21 | + // Handle the case where the iterable is empty |
| 22 | + resolve([]); |
| 23 | + } |
| 24 | + |
| 25 | + // Iterate through all promises |
| 26 | + promises.forEach((promise, index) => { |
| 27 | + // Handle a promise or value; use Promise.resolve to wrap non-promise values |
| 28 | + Promise.resolve(promise) |
| 29 | + .then((value) => { |
| 30 | + // Store the resolved value at the corresponding index |
| 31 | + results[index] = value; |
| 32 | + completed += 1; |
| 33 | + |
| 34 | + // If all promises are resolved, resolve the result |
| 35 | + if (completed === promises.length) { |
| 36 | + resolve(results); |
| 37 | + } |
| 38 | + }) |
| 39 | + .catch((error) => { |
| 40 | + // If any promise rejects, reject the entire promise |
| 41 | + reject(error); |
| 42 | + }); |
| 43 | + }); |
| 44 | + }); |
| 45 | +} |
| 46 | +``` |
| 47 | +### Explanation |
| 48 | +#### Validate Input: |
| 49 | + |
| 50 | +* Check if the input iterable is an object and has the iterator protocol `(Symbol.iterator)`. If not, throw a TypeError. |
| 51 | +#### Handle Empty Input: |
| 52 | + |
| 53 | +* If the iterable is empty, resolve immediately with an empty array ([]). |
| 54 | +#### Iterate Over Promises: |
| 55 | + |
| 56 | +* Convert the iterable into an array using Array.from(iterable) for uniform handling. |
| 57 | +* Iterate over the array of promises and wrap all values using Promise.resolve. Non-promise values are converted to promises that resolve immediately. |
| 58 | +#### Track Resolutions: |
| 59 | + |
| 60 | +* Maintain an array (results) to store resolved values at the correct indices. |
| 61 | +* Use a counter (completed) to track how many promises have successfully resolved. |
| 62 | +#### Resolve When All Promises Succeed: |
| 63 | + |
| 64 | +* Once the completed counter equals the total number of promises, resolve the final promise with the results array. |
| 65 | +#### Reject on Any Failure: |
| 66 | + |
| 67 | +* If any promise rejects, immediately reject the resulting promise with the error. |
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