JavaScript 是一種單執行緒程式語言，這表示它一次只能執行一個任務。對於諸如獲取資料或設定計時器之類的非同步操作來說，這變得很棘手，這可能會阻塞執行流程並減慢您的應用程式的速度。
為了在不凍結線程的情況下處理這些非同步任務，我們遇到了Promise——一個簡化非同步程式設計的強大工具。透過Promises，您可以更有效地管理長時間運行的任務，編寫更乾淨、更具可讀性的程式碼，並避免可怕的「回調地獄。」
在本文中，我的目標是讓您熟悉 Promise 是什麼、它們如何運作以及它們如何簡化非同步程式設計。
想像一下您正在餐廳點餐。下訂單後，您無需在廚房等待食物準備好。相反，您可以在廚房在後台準備餐點的同時繼續交談或享受氛圍。餐廳承諾一旦食物準備好就會為您提供食物。您可以相信這個承諾，因為最終會發生以下兩種情況之一：您的餐食將到達（已完成），或者廚房會通知您他們無法完成訂單（已拒絕） ）。
在 JavaScript 中，Promise 以類似的方式運作。當您要求 JavaScript 執行一些需要時間的操作（例如從伺服器取得資料）時，它會傳回一個 Promise。這個Promise不會立即給你結果。相反，它會告訴您，「工作完成後我會回覆您。」在此期間，其餘程式碼將繼續運行。任務完成後，Promise 為：
Promise 代表一個可能現在、未來或永遠無法使用的值。它有三種狀態：
要建立 Promise，可以使用 Promise 建構函數，它採用一個具有兩個參數的函數（稱為執行器）：resolve 和reject。當 Promise 滿足時，將呼叫resolve函數，而當被拒絕時，將呼叫reject函數。
const myPromise = new Promise((resolve, reject) => {
// Simulating an asynchronous task (e.g., fetching data)
const success = true; // Simulate success or failure
if (success) {
resolve("Operation completed successfully!"); // Fulfill the promise
} else {
reject("Operation failed."); // Reject the promise
}
});
一旦建立了Promise，您可以透過呼叫resolve或reject來決定其結果：
建立 Promise 後，下一步就是使用它。 JavaScript 提供了幾種處理 Promise 結果的方法：.then()、.catch() 和 .finally()。這些方法中的每一個都有特定的目的，並允許您有效地管理非同步操作的結果。
const fetchData = () => {
return new Promise((resolve) => {
setTimeout(() => {
resolve("Data fetched successfully!");
}, 1000);
});
};
fetchData()
.then(result => {
console.log(result); // Logs: Data fetched successfully!
});
const fetchWithError = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
reject("Error fetching data."); // Simulating an error
}, 1000);
});
};
fetchWithError()
.then(result => {
console.log(result);
})
.catch(error => {
console.error(error); // Logs: Error fetching data.
});
fetchData()
.then(result => {
console.log(result); // Logs: Data fetched successfully!
})
.catch(error => {
console.error(error); // Handle error
})
.finally(() => {
console.log("Promise has settled."); // Logs after either success or failure
});
簡而言之：
One of the most powerful features of Promises is their ability to be chained together, allowing you to perform multiple asynchronous operations in sequence. This means each operation waits for the previous one to complete before executing, which is particularly useful when tasks depend on each other.
Let's take a look at the following example:
const fetchUserData = () => {
return new Promise((resolve) => {
setTimeout(() => {
resolve({ userId: 1, username: "JohnDoe" });
}, 1000);
});
};
const fetchPosts = (userId) => {
return new Promise((resolve) => {
setTimeout(() => {
resolve(["Post 1", "Post 2", "Post 3"]); // Simulated posts
}, 1000);
});
};
// Chaining Promises
fetchUserData()
.then(user => {
console.log("User fetched:", user);
return fetchPosts(user.userId); // Pass userId to the next promise
})
.then(posts => {
console.log("Posts fetched:", posts);
})
.catch(error => {
console.error("Error:", error);
});
In this example, the fetchUserData function returns a Promise that resolves with user information. The resolved value is then passed to the fetchPosts function, which returns another Promise. If any of these Promises are rejected, the error is caught in the final .catch() method, allowing for effective error handling throughout the chain.
In conclusion, Promises are a crucial part of modern JavaScript, enabling developers to handle asynchronous operations in a more structured and efficient way. By using Promises, you can:
As you implement Promises in your own projects, you'll find that they not only improve code readability but also enhance the overall user experience by keeping your applications responsive. I hope that this journey through JavaScript's foundational concepts has provided valuable insights for developers. Happy coding!
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