NodeJs-promise and grammar async_await

Callback hell callback region

When we write code to synchronize time-consuming way, it will clog JS single-threaded, causing the CPU has been waiting for IO to complete before going to execute the code behind.

The execution speed of the CPU is much larger than the hard disk IO speed, so that the wait will only result in a waste of resources.

Asynchronous IO is to solve this problem, can not let the asynchronous CPU idle as much as possible, it will not be completed in that waiting for IO; but is passed to the bottom of the event loop a function of their own to execute the following code. Once the disk IO is completed, the function will be executed as a notification.

Although asynchronous and callback programming method can make full use of CPU, but when the code logic becomes more complex, new problems have emerged - Callback callback geographical Hell!

Function Code

Achieve the following functions:

1. Determine a file is a file or directory;
2. If it is a directory, read files in this directory, find the end of a txt file;
3. Get these txt file size.

Asynchronously

function withoutPromise() {
    let target = "test";
    fs.stat(target, (err, stat)=>{
        if(err){
            throw err;
        }
        // 如果是文件夹
        if(stat.isDirectory()){
            fs.readdir(target, (err, files)=>{
                // 遍历files
                files.forEach( f =>{
                    if(path.extname(f) === '.txt'){
                        fs.stat(path.join(target, f), (err, stat)=>{
                            console.log(f+ " : "+stat.size);
                        });
                    }
                } );
            });
        }
    });
}

Promise way

async function withPromise() {
    let target = "test";
    //将fs.stat转为一个可以返回Promise对象的方法
    let pstat = util.promisify(fs.stat);
    let stat = await pstat(target);
    // 如果是文件夹
    if(stat.isDirectory()){
        //将fs.readdir转为一个可以返回Promise对象的方法
        let preaddir = util.promisify(fs.readdir)
        let files = await preaddir(target)
        files.forEach( async (f) => {
            if(path.extname(f) === '.txt'){
                let stat = await pstat(path.join(target, f));
                console.log(stat.size);
            }
        });
    }
}

Promise和async/await

PromiseAnd async/awaitthat is to solve the Callback hell of a problem.

promise

promiseThe role is an asynchronous callback code packaged, the original of a callback function callback function is split into two, so that the benefits are more readable. The syntax is as follows:

// 创建promise对象
let promise = new Promise((resolve, reject)=>{
    // 在异步操作成功的情况选调用resolve，失败的时候调用reject
    fs.readFile('xxx.txt',(err, data)=>{
        if(err){
            reject(err)
        }else {
            resolve(data.toString())
        }
    })
});
// 使用promise
promise.then((text)=>{
    //then方法是当Promise内部调用了resolve的时候执行
}).catch((err)=>{
    //catch方法是当Promise内部调用了reject的时候执行
    console.log(err);
})

• Syntax Note: internal Promise resolve and reject method can only be called once, you can not call this a call that; if the call is invalid.

async/await

async/awaitThe direct effect is the Promise asynchronous code into a synchronized writing, note that the code still is asynchronous .

Syntax requirements:

• awaitIt can only be used in the asyncmodified approach, but there is asyncnot necessarily required await.
• awaitWith only behind asyncand methods promise.

Assuming that the object has a promise, now use the async / await write:

async function asyncDemo() {
    try {
        // 当promise的then方法执行的时候
        let text = await promise
        // 当你用promise包装了所有的异步回调代码后，就可以一直await，真正意义实现了以同步的方式写异步代码
        console.log('异步道明执行');
    }catch (e){
        // 捕获到promise的catch方法的异常
        console.log(e);
    }
}
asyncDemo()
console.log('我是同步代码');

Ultimate writing asynchronous code

1. Use first promisepackage asynchronous callback code, node may be used to provide util.promisifya method;
2. Use async/awaitwriting asynchronous code.

reference

• Dark Horse programmer Vincent stack 120 block chain to develop open-source tutorials

  https://github.com/itheima1/BlockChain

Learning Videos

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