HowProgrammingWorks
diff --git a/‎JavaScript/0-async.js‎
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+'use strict';
+
+class AsyncMutex {
+  #queue = [];
+  #held = false;
+
+  enter() {
+    if (!this.#held) {
+      this.#held = true;
+      return Promise.resolve();
+    }
+    return new Promise((resolve) => {
+      this.#queue.push(resolve);
+    });
+  }
+
+  leave() {
+    const next = this.#queue.shift();
+    if (next) return void queueMicrotask(next);
+    this.#held = false;
+  }
+}
+
+class Account {
+  #mutex = new AsyncMutex();
+  #balance = 0;
+
+  get balance() {
+    return this.#balance;
+  }
+
+  async deposit(amount, workTime) {
+    await this.#mutex.enter();
+    try {
+      const before = this.#balance;
+      await delay(workTime);
+      this.#balance = before + amount;
+      return this.#balance;
+    } finally {
+      this.#mutex.leave();
+    }
+  }
+}
+
+const delay = (ms) =>
+  new Promise((resolve) => setTimeout(resolve, ms));
+
+const main = async () => {
+  const account = new Account();
+
+  await Promise.all([
+    account.deposit(10, 50),
+    account.deposit(20, 10),
+    account.deposit(30, 5),
+  ]);
+
+  console.log(account.balance);
+};
+
+main().catch(console.error);