How to use async function in js
This time I will show you how to use the async function in js, and what are the precautions for using the async function in js. The following is a practical case, let's take a look.
1. The ultimate solution
Asynchronous operation is a troublesome thing in From the earliest Isn’t asynchronous I/O just reading a file? Why does it have to be so complicated? The highest state of asynchronous programming is that you don't have to worry about whether it is asynchronous at all. async functions are the light at the end of the tunnel, considered by many to be the ultimate solution for asynchronous operations.2. What is the async function?
In a word, async function is the syntactic sugar of Generator function. There is a Generator function in the previous article, which reads two files in sequence.var fs = require('fs');
var readFile = function (fileName){
return new Promise(function (resolve, reject){
fs.readFile(fileName, function(error, data){
if (error) reject(error);
resolve(data);
});
});
};
var gen = function* (){
var f1 = yield readFile('/etc/fstab');
var f2 = yield readFile('/etc/shells');
console.log(f1.toString());
console.log(f2.toString());
};var asyncReadFile = async function (){
var f1 = await readFile('/etc/fstab');
var f2 = await readFile('/etc/shells');
console.log(f1.toString());
console.log(f2.toString());
};3. Advantages of async function
The improvements of async function to Generator function are reflected in the following three points. (1) Built-in executor. The execution of the Generator function must rely on the executor, so there is the co function library, and the async function comes with its own executor. In other words, the execution of async functions is exactly the same as ordinary functions, with only one line.var result = asyncReadFile();
expression needs to wait for the result.
(3) Wider applicability. According to the co function library convention, the yield command can only be followed by a Thunk function or a Promise object, while the await command of an async function can be followed by a Promise object and primitive type values (numeric values,stringsand Boolean values, but This is equivalent to a synchronous operation).
4. Implementation of async function
The implementation of async function is to wrap the Generator function and the automatic executor in a function.async function fn(args){
// ...
}
// 等同于
function fn(args){
return spawn(function*() {
// ...
});
}function spawn(genF) {
return new Promise(function(resolve, reject) {
var gen = genF();
function step(nextF) {
try {
var next = nextF();
} catch(e) {
return reject(e);
}
if(next.done) {
return resolve(next.value);
}
Promise.resolve(next.value).then(function(v) {
step(function() { return gen.next(v); });
}, function(e) {
step(function() { return gen.throw(e); });
});
}
step(function() { return gen.next(undefined); });
});
}5. Usage of async function
Same as the Generator function, the async function returns a Promise object, and you can use the then method to add a callback function. When the function is executed, once it encounters await, it will return first, wait until the triggered asynchronous operation is completed, and then execute the subsequent statements in the function body. Below is an example.async function getStockPriceByName(name) {
var symbol = await getStockSymbol(name);
var stockPrice = await getStockPrice(symbol);
return stockPrice;
}
getStockPriceByName('goog').then(function (result){
console.log(result);
});function timeout(ms) {
return new Promise((resolve) => {
setTimeout(resolve, ms);
});
}
async function asyncPrint(value, ms) {
await timeout(ms);
console.log(value)
}
asyncPrint('hello world', 50);6. Notes
The Promise object behind the await command may result in rejected, so it is best to put the await command in try ...catch code block.async function myFunction() {
try {
await somethingThatReturnsAPromise();
} catch (err) {
console.log(err);
}
}
// 另一种写法
async function myFunction() {
await somethingThatReturnsAPromise().catch(function (err){
console.log(err);
});
}async function dbFuc(db) {
let docs = [{}, {}, {}];
// 报错
docs.forEach(function (doc) {
await db.post(doc);
});
}上面代码会报错，因为 await 用在普通函数之中了。但是，如果将 forEach 方法的参数改成 async 函数，也有问题。
async function dbFuc(db) {
let docs = [{}, {}, {}];
// 可能得到错误结果
docs.forEach(async function (doc) {
await db.post(doc);
});
}上面代码可能不会正常工作，原因是这时三个 db.post 操作将是并发执行，也就是同时执行，而不是继发执行。正确的写法是采用 for 循环。
async function dbFuc(db) {
let docs = [{}, {}, {}];
for (let doc of docs) {
await db.post(doc);
}
}如果确实希望多个请求并发执行，可以使用 Promise.all 方法。
async function dbFuc(db) {
let docs = [{}, {}, {}];
let promises = docs.map((doc) => db.post(doc));
let results = await Promise.all(promises);
console.log(results);
}
// 或者使用下面的写法
async function dbFuc(db) {
let docs = [{}, {}, {}];
let promises = docs.map((doc) => db.post(doc));
let results = [];
for (let promise of promises) {
results.push(await promise);
}
console.log(results);
}相信看了本文案例你已经掌握了方法，更多精彩请关注php中文网其它相关文章！
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