feat: implement Promise.all()

32.implement-Promise.all.md

@@ -0,0 +1,102 @@
+# 32. implement `Promise.all()`
+Promise.all is a method in JavaScript used to handle multiple promises concurrently. It takes an iterable (such as an array) of promises and returns a single promise. This returned promise resolves when all the promises in the iterable have resolved, or rejects if any of the promises in the iterable reject.
+
+### Key Features of `Promise.all`
+
+1. **Concurrent Execution**:
+
+    * `Promise.all` runs multiple promises in parallel.
+    * It waits for all the promises to settle (either resolved or rejected).
+2. **Resolved Value**:
+
+    * When all promises resolve, `Promise.all` resolves with an array of their results, maintaining the order of the original promises.
+    * Example: `Promise.all([promise1, promise2]).then((values) => { console.log(values); });`
+    * If `promise1` resolves to `value1` and `promise2` resolves to `value2`, the output will be `[value1, value2]`.
+3. **Rejection Handling**:
+
+    * If any promise in the iterable rejects, `Promise.all` immediately rejects with the reason of the first promise that rejected.
+    * Example: `Promise.all([promise1, promise2]).catch((error) => { console.log(error); });`
+    * If `promise1` rejects with an `error1`, `Promise.all` will reject with `error1`.
+
+
+### Problem
+
+https://bigfrontend.dev/problem/implement-Promise-all
+
+#
+
+### Problem Description
+
+> The Promise.all() method takes an iterable of promises as an input, and returns a single Promise that resolves to an array of the results of the input promises
+
+source - [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/all)
+
+Could you write your own `all()` ? which should works the same as `Promise.all()`
+
+#
+
+### Solution
+
+```js
+/**
+ * @param {Array<any>} promises - notice input might have non-Promises
+ * @return {Promise<any[]>}
+ */
+function all(promises) {
+  if (promises.length === 0) {
+    return Promise.resolve([]);
+  }
+
+  const results = Array(promises.length);
+  let count = 0;
+
+  return new Promise((resolve, reject) => {
+    promises.forEach((promise, i) => {
+      if (!(promise instanceof Promise)) {
+        promise = Promise.resolve(promise);
+      }
+
+      promise
+        .then((res) => {
+          results[i] = res;
+          count++;
+
+          if (count === promises.length) {
+            resolve(results);
+          }
+        })
+        .catch((err) => {
+          reject(err);
+        });
+    });
+  });
+}
+```
+
+### Example Usage
+
+```javascript
+const promise1 = Promise.resolve(3);
+const promise2 = new Promise((resolve, reject) => setTimeout(resolve, 100, 'foo'));
+const promise3 = fetch('https://api.example.com/data');
+
+Promise.all([promise1, promise2, promise3])
+  .then((values) => {
+    console.log(values); // [3, 'foo', Response object from fetch]
+  })
+  .catch((error) => {
+    console.error('One of the promises failed:', error);
+  });
+```
+
+### Use Cases
+
+* **Aggregating Results**: When you need to wait for multiple asynchronous operations to complete before proceeding, such as fetching data from multiple sources.
+* **Parallel Execution**: When operations do not depend on each other and can be executed in parallel for efficiency.
+
+### Important Notes
+
+* If the iterable passed to `Promise.all` is empty, it returns a promise that resolves to an empty array.
+* The order of results in the resolved array corresponds to the order of the promises passed to `Promise.all`.
+
+By leveraging `Promise.all`, developers can handle multiple asynchronous operations efficiently, ensuring that subsequent code execution waits until all specified promises have been settled.