How to use async in javascript

As web applications become more complex, asynchronous programming becomes more and more important. In JavaScript, we can use the async/await keyword to manage asynchronous operations. This article will introduce the basic usage of async and provide some examples to help you understand better.

What is async?

The async function is a new asynchronous programming method introduced in ES6. The async keyword can turn a JavaScript function into an asynchronous function, making the function return a Promise object and making it more concise and clear when handling asynchronous operations. Async functions are often used with the await keyword, which pauses the execution of the async function until the Promise is completed.

The syntax of the async function is as follows:

async function functionName() {
    //异步操作
}

Using async

When using async functions, you usually need to use Promise objects in combination. For example, if we want to obtain a piece of remote data, we can use the fetch function:

async function fetchData() {
    const response = await fetch('http://example.com/data');
    const data = await response.json();
    return data;
}

In the above code, we create an async function named fetchData and use the fetch function to initiate a remote ask. Since the fetch function is an asynchronous operation, we need to use the await keyword to wait for the completion of the Promise object. We can use destructuring assignment to store the obtained data in the variable data and return it after the Promise returned by await is completed.

Handling multiple Promise

When we need to process multiple Promise objects, we can use the Promise.all() method. The Promise.all() method combines Promise objects into a Promise object and returns an array containing the results of each Promise object when all Promise objects succeed. If any of the Promise objects fails, this method will immediately return a failed Promise object and no longer wait for the outstanding Promise object.

For example, if we want to get data from two remote data sources at the same time and operate after they are both obtained, we can operate like the following:

async function fetchData() {
    const [data1, data2] = await Promise.all([(async () => {
        const response = await fetch('http://example.com/data1');
        return response.json();
    })(), (async () => {
        const response = await fetch('http://example.com/data2');
        return response.json();
    })()]);
    console.log(data1, data2);
}

In the above code , we created an async function named fetchData, and used the Promise.all() method in it to wait for data from two remote data sources. We used a self-executing function and the async/await keyword to retrieve two data sources and form them into an array of Promise objects. When the data from both data sources has been fully parsed, we store them in variables data1 and data2 and do simple logging on the console.

Handling Errors

When handling errors in async functions, we usually use try/catch statements. Since the async function returns a Promise object, when we throw an exception, the returned Promise object will be rejected. We can catch this exception in catch block and handle it.

For example, we may encounter an exception where the network request fails, as shown below:

async function fetchData() {
    try {
        const response = await fetch('http://example.com/data');
        const data = await response.json();
        return data;
    } catch (error) {
        console.error(error);
    }
}

In the above code, we created an async function named fetchData and used try/catch statements to handle exceptions when network requests fail. If the request is successful, we store the data in the variable data and return it. Otherwise, we print the error to the console.

Conclusion

The async/await keyword is a convenient way for JavaScript to manage asynchronous programming. Using async/await results in cleaner code that is easier to read and understand. When you need to manage multiple asynchronous operations or handle errors, you can also use Promise and try/catch statements to achieve this. Hope this article is helpful to you!

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