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ES2017 The standard has been established in 2017 year 6 It was officially finalized in September , And broad support for the latest features : An asynchronous function . If you've ever been asynchronous JavaScript The logic perplexity of , This new function is designed for you .

Asynchronous functions more or less allow you to write some sequential JavaScript Code , But it doesn't have to be callbacks、generators or promise It contains your logic .

The following code :

function logger() {
let data = fetch('http://sampleapi.com/posts')
console.log(data)
}
logger()

This code doesn't live up to your expectations . If you are JS Writing in the , Then you may know why .

Here's the code , But it did what you expected .

async function logger() {
let data = await fetch('http:sampleapi.com/posts')
console.log(data)
}
logger()

This code works , Intuitively , Just a lot async and await Two words .

ES6 Before the standard JavaScript An asynchronous function

In-depth learning async and await Before , We need to understand Promise. In order to understand Promise, We need to go back to the normal callback function to learn more .

Promise Is in ES6 Introduced in , And promote the writing of JavaScript Asynchronous code aspect of , It's a huge improvement . Since then, writing callback functions is not so painful .

A callback is a function , You can pass the result to a function and call it inside that function , In response to an event . meanwhile , This is also JS The basis of .

function readFile('file.txt', (data) => {
// This is inside the callback function
console.log(data)
}

This function simply records data to a file , It's impossible to read a file before it's finished . The process seems simple , But if you want to read and record five different files in order , How does it need to be implemented ?

No, Promise When , In order to perform tasks in sequence , It needs to be implemented through nested callbacks , Like the following code :

// This is officially callback hell
function combineFiles(file1, file2, file3, printFileCallBack) {
let newFileText = ''
readFile(string1, (text) => {
newFileText += text
readFile(string2, (text) => {
newFileText += text
readFile(string3, (text) => {
newFileText += text
printFileCallBack(newFileText)
}
}
}
}

It's hard to infer what's going on under the function , At the same time, it's hard to deal with errors in various scenarios , For example, if one of the files does not exist .

Promise Improved the situation

That's exactly what it is. Promise The advantage of ,Promise It's a kind of reasoning about data that hasn't been generated yet .Kyle Simpson take Promise Interpreted as : It's like ordering in a fast food restaurant .

  • Order
  • Pay for the lunch you ordered , And get the queue number
  • Waiting for lunch
  • When your lunch is ready , I'll call your order number to remind you to pick up your meal
  • Got lunch

Like the scene above , When you're waiting for a meal , You can't have lunch , But you can prepare for lunch ahead of time . You can do other things , Now you know that lunch is coming , Even though you can't enjoy it yet , But this lunch has “promise” Here you are . That's what's called promise, An object that represents data that will eventually exist .

readFile(file1)
.then((file1-data) => { /* do something */ })
.then((previous-promise-data) => { /* do the next thing */ })
.catch( /* handle errors */ )

It's on it Promise  grammar . Its main advantage is that it can link queue events together in an intuitive way . Although this example is clear and easy to understand , But there are callbacks .Promise It just makes callbacks simpler and more intuitive .

best mode :async / await

A few years ago ,async The function incorporates JavaScript The ecological system . Just last month ,async The function becomes JavaScript The official nature of language , And there's a lot of support .

async and await It's based on Promise and generator On . Essentially , Allow us to use await This keyword pauses anywhere we want in any function .

async function logger() {
// pause until fetch returns
let data = await fetch('http://sampleapi.com/posts')
console.log(data)
}

After the above code runs , Get the results you want . Code from API Data is recorded in the call .

The advantage of this approach is that it's very intuitive . The way you write code is the way your brain thinks , Tell the script to pause where it needs to .

Another benefit is , When we can't use promise when , You can also use try and catch:

async function logger () {
try {
let user_id = await fetch('/api/users/username')
let posts = await fetch('/api/`${user_id}`')
let object = JSON.parse(user.posts.toString())
console.log(posts)
} catch (error) {
console.error('Error:', error)
}
}

The above is an example of deliberate error , To prove a point : At run time ,catch You can catch errors in any step . There are at least three places ,try May fail , This is one of the cleanest ways to handle errors in asynchronous code .

We can also use... With loops and conditions async function :

async function count() {
let counter = 1
for (let i = 0; i < 100; i++) {
counter += 1
console.log(counter)
await sleep(1000)
}
}

This is a very simple example , If you run this program , You'll see the code in sleep Pause on call , The next iteration of the loop will take place in 1 Start in seconds .

Key points and details

I believe we have felt asyns and await The beauty of , Let's take a closer look at the details :

  • async and await Based on the Promise above . Use async, Always return one Promise. Remember that , Because it's also a mistake prone place .
  • When executed await when , The program pauses the current function , Not all the code
  • async and await It's non blocking
  • Still usable Promise helpers, for example Promise.all( )

As in the previous example :

async function logPosts () {
try {
let user_id = await fetch('/api/users/username')
let post_ids = await fetch('/api/posts/<code>${user_id}')
let promises = post_ids.map(post_id => {
return fetch('/api/posts/${post_id}')
}
let posts = await Promise.all(promises)
console.log(posts)
} catch (error) {
console.error('Error:', error)
}
}
  • await Can only be used to declare as async The function of
  • therefore , Cannot use... In a global context await

The following code :

// throws an error
function logger (callBack) {
console.log(await callBack)
} // works!
async function logger () {
console.log(await callBack)
}

Now available

To 2017 year 6 month , Almost all browsers can use async and await. To make sure your code is always available , You need to use Babel Put your JavaScript The code compiles to a syntax that old browsers also support .

If it's for more ES2017 I'm interested in the content , Please visit ES2017 A complete list of features .

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