Merge branch 'master' of github.com:javascript-tutorial/en.javascript.info into sync-035c5267

Author: iliakan

1-js/11-async/08-async-await/04-promise-all-failure/solution.md

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+
+The root of the problem is that `Promise.all` immediately rejects when one of its promises rejects, but it do nothing to cancel the other promises.
+
+In our case, the second query fails, so `Promise.all` rejects, and the `try...catch` block catches this error.Meanwhile, other promises are *not affected* - they independently continue their execution. In our case, the third query throws an error of its own after a bit of time. And that error is never caught, we can see it in the console.
+
+The problem is especially dangerous in server-side environments, such as Node.js, when an uncaught error may cause the process to crash.
+
+How to fix it?
+
+An ideal solution would be to cancel all unfinished queries when one of them fails. This way we avoid any potential errors.
+
+However, the bad news is that service calls (such as `database.query`) are often implemented by a 3rd-party library which doesn't support cancellation. Then there's no way to cancel a call.
+
+As an alternative, we can write our own wrapper function around `Promise.all` which adds a custom `then/catch` handler to each promise to track them: results are gathered and, if an error occurs, all subsequent promises are ignored.
+
+```js
+function customPromiseAll(promises) {
+  return new Promise((resolve, reject) => {
+    const results = [];
+    let resultsCount = 0;
+    let hasError = false; // we'll set it to true upon first error
+
+    promises.forEach((promise, index) => {
+      promise
+        .then(result => {
+          if (hasError) return; // ignore the promise if already errored
+          results[index] = result;
+          resultsCount++;
+          if (resultsCount === promises.length) {
+            resolve(results); // when all results are ready - successs
+          }
+        })
+        .catch(error => {
+          if (hasError) return; // ignore the promise if already errored
+          hasError = true; // wops, error!
+          reject(error); // fail with rejection
+        });
+    });
+  });
+}
+```
+
+This approach has an issue of its own - it's often undesirable to `disconnect()` when queries are still in the process.
+
+It may be important that all queries complete, especially if some of them make important updates.
+
+So we should wait until all promises are settled before going further with the execution and eventually disconnecting.
+
+Here's another implementation. It behaves similar to `Promise.all` - also resolves with the first error, but waits until all promises are settled.
+
+```js
+function customPromiseAllWait(promises) {
+  return new Promise((resolve, reject) => {
+    const results = new Array(promises.length);
+    let settledCount = 0;
+    let firstError = null;
+
+    promises.forEach((promise, index) => {
+      Promise.resolve(promise)
+        .then(result => {
+          results[index] = result;
+        })
+        .catch(error => {
+          if (firstError === null) {
+            firstError = error;
+          }
+        })
+        .finally(() => {
+          settledCount++;
+          if (settledCount === promises.length) {
+            if (firstError !== null) {
+              reject(firstError);
+            } else {
+              resolve(results);
+            }
+          }
+        });
+    });
+  });
+}
+```
+
+Now `await customPromiseAllWait(...)` will stall the execution until all queries are processed.
+
+This is a more reliable approach, as it guarantees a predictable execution flow.
+
+Lastly, if we'd like to process all errors, we can use either use `Promise.allSettled` or write a wrapper around it to gathers all errors in a single [AggregateError](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/AggregateError) object and rejects with it.
+
+```js
+// wait for all promises to settle
+// return results if no errors
+// throw AggregateError with all errors if any
+function allOrAggregateError(promises) {
+  return Promise.allSettled(promises).then(results => {
+    const errors = [];
+    const values = [];
+
+    results.forEach((res, i) => {
+      if (res.status === 'fulfilled') {
+        values[i] = res.value;
+      } else {
+        errors.push(res.reason);
+      }
+    });
+
+    if (errors.length > 0) {
+      throw new AggregateError(errors, 'One or more promises failed');
+    }
+
+    return values;
+  });
+}
+```

1-js/11-async/08-async-await/04-promise-all-failure/task.md

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+
+# Dangerous Promise.all
+
+`Promise.all` is a great way to parallelize multiple operations. It's especially useful when we need to make parallel requests to multiple services.
+
+However, there's a hidden danger. We'll see an example in this task and explore how to avoid it.
+
+Let's say we have a connection to a remote service, such as a database.
+
+There're two functions: `connect()` and `disconnect()`.
+
+When connected, we can send requests using `database.query(...)` - an async function which usually returns the result but also may throw an error.
+
+Here's a simple implementation:
+
+```js
+let database;
+
+function connect() {
+  database = {
+    async query(isOk) {
+      if (!isOk) throw new Error('Query failed');
+    }
+  };
+}
+
+function disconnect() {
+  database = null;
+}
+
+// intended usage:
+// connect()
+// ...
+// database.query(true) to emulate a successful call
+// database.query(false) to emulate a failed call
+// ...
+// disconnect()
+```
+
+Now here's the problem.
+
+We wrote the code to connect and send 3 queries in parallel (all of them take different time, e.g. 100, 200 and 300ms), then disconnect:
+
+```js
+// Helper function to call async function `fn` after `ms` milliseconds
+function delay(fn, ms) {
+  return new Promise((resolve, reject) => {
+    setTimeout(() => fn().then(resolve, reject), ms);
+  });
+}
+
+async function run() {
+  connect();
+
+  try {
+    await Promise.all([
+      // these 3 parallel jobs take different time: 100, 200 and 300 ms
+      // we use the `delay` helper to achieve this effect
+*!*
+      delay(() => database.query(true), 100),
+      delay(() => database.query(false), 200),
+      delay(() => database.query(false), 300)
+*/!*
+    ]);
+  } catch(error) {
+    console.log('Error handled (or was it?)');
+  }
+
+  disconnect();
+}
+
+run();
+```
+
+Two of these queries happen to be unsuccessful, but we're smart enough to wrap the `Promise.all` call into a `try..catch` block.
+
+However, this doesn't help! This script actually leads to an uncaught error in console!
+
+Why? How to avoid it?