ChatGPT helps me learn [js-async/await]

This question involves asking about usage, questioning doubts, and confirming mastery~

Question

js sync await Tell me, I’m still a little confused

In JavaScript, we often need to handle asynchronous tasks, such as getting data from the server, sending data to the server, etc. The traditional way is to use callback functions to handle asynchronous tasks, which will lead to callback hell when nested at multiple levels, making the code difficult to maintain and debug. To solve this problem, JavaScript introduced Promise and async/await to handle asynchronous tasks.

Among them, async/await is a new syntactic sugar in ES8, which can handle asynchronous tasks more simply and intuitively. Regarding async/await, you can understand it from the following points:

1. The async function is an asynchronous function that returns a Promise object. You can use the await keyword to wait for the result of an asynchronous operation. While waiting, During the process, the async function will release the CPU so that other tasks can be executed.

2. await keyword can only be used in async functions. You can wait for the result of a Promise object. If the Promise object becomes resolved, the value of the await expression is the result returned by Promise. ; If the Promise object becomes rejected, an error will be thrown and caught by the try-catch block.

3. There can be multiple await expressions inside the async function, and they are executed sequentially. The subsequent await expressions need to wait for the previous await expression to complete before executing.

4. Using async/await can write asynchronous code in a form similar to synchronous code. The code is more readable and understandable, and easier to debug and maintain.

The following is an example of using async/await to handle asynchronous tasks:

async function getData() {
  try {
    const result1 = await fetch('/api/data1'); // 等待获取数据1
    const result2 = await fetch('/api/data2'); // 等待获取数据2
    return [result1, result2]; // 返回获取的数据
  } catch (error) {
    console.error(error); // 捕获错误并输出到控制台
  }
}

getData().then(data => {
  console.log(data); // 输出获取的数据
});

In the above example, the getData function is declared using the async keyword and inside the function Use the await keyword to wait for the result of an asynchronous operation. Here, two await expressions are used to obtain different data. In this example, the result arrays of two asynchronous operations are eventually returned. If an exception occurs, it will be caught by the try-catch block and error information will be output. Finally, we get the asynchronous data by calling the getData() method, and output the data to the console after returning the result.

Raise doubts

I feel very confused. await is to wait for the corresponding asynchronous function, so why should we add async to the function that uses await?

Very good question! The answer is: because the await keyword can only be used in async functions.

When using an asynchronous function to obtain the result of an asynchronous task, we need to place it inside an async function, and then use await to wait for the result of the asynchronous operation. Therefore, if you want to use the await keyword, the function must be declared as an async function. On the other hand, if you are not using an async function, you cannot use await in that function.

In other words, only when a function is declared as an async function, the function can contain the await keyword, which is also the basic premise of the async/await syntax. Declaring a function as async tells the JS engine that the function may return a Promise, allowing the JS engine to perform special processing when executing the function.

It should be noted that using await to wait for the result of an asynchronous task does not necessarily require returning the Promise object immediately inside the function, because the execution process of the await keyword is related to the Promise chain call. The execution process is similar, and Promise objects are automatically created and returned.

In general, async and await are the syntactic sugar that constitutes the Promise asynchronous programming style. They make asynchronous code more intuitive and readable, allowing us to use synchronous style code to write and process asynchronous operation code.

Confirm understanding

Oh, actually the async mark does not modify the function, right? I originally thought that this mark would turn a function into an asynchronous function for execution

You are right! In fact, the async keyword does not directly turn a function into an asynchronous function execution. It simply tells the JavaScript engine that there may be asynchronous operations inside the function, and the execution of the function may need to be paused when using the await keyword.

In other words, the async keyword does not change the nature of the function, but provides a more convenient way to handle asynchronous operations. When the async function is called, it returns a Promise object, and the code inside the async function is automatically encapsulated into the resolve method of the Promise object at runtime. When you use the await keyword inside an async function to wait for an asynchronous operation to complete, the function will pause execution until the asynchronous operation returns a result or throws an error.

In addition, it should be noted that when using the return statement to return a value inside an async function, the returned value will be packaged into a Promise object by Promise.resolve(), and the value of the Promise object is the return statement. value. If an exception is thrown inside the async function, it will be wrapped into a Promise object by Promise.reject() and returned to the caller.

Therefore, we can use async/await with Promise-style code to create a simpler and more intuitive asynchronous programming model.

Further confirmation of knowledge points

So async is a representation of processing asynchronous operations, not a syntax for creating asynchronous operations

Yes, your understanding is very accurate. async is a representation for handling asynchronous operations, not a syntax for creating asynchronous operations.

In fact, JavaScript already has some mechanisms for handling asynchronous operations, such as setTimeout, XMLHttpRequest and fetch. These mechanisms are based on the asynchronous programming model of the event loop and are widely used in various JavaScript applications.

However, these mechanisms have some problems, such as callback hell and poor code readability. In order to solve these problems, Promise and async/await were introduced as a new asynchronous programming method and became widely used quickly.

In this process, the async function plays an important role in processing asynchronous operations. It provides a more intuitive and easy-to-understand code form, and enables JavaScript developers to better master asynchronous programming technology. Therefore, we can think of async functions as a complement to the Promise-style code model to achieve more flexible and efficient asynchronous programming.

A good inquiry~                                                                                                

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