JavaScript Async 异步请求
Await 语法
函数前的关键字 await 使函数等待 promise：
let value = await promise;
await 关键字只能在 async 函数中使用。
实例
让我们来学习如何使用它。
async function getFile() {
let promise = new Promise(function(resolve, reject) {
let req = new XMLHttpRequest();
req.open('GET', "mycar.html");
req.onload = function() {
if (req.status == 200) {resolve(req.response);}
else {resolve("File not Found");}
};
req.send();
});
let result = await promise;
document.getElementById("demo").innerHTML = result;
}
getFile();
let promise = new Promise(function(resolve, reject) {
let req = new XMLHttpRequest();
req.open('GET', "mycar.html");
req.onload = function() {
if (req.status == 200) {resolve(req.response);}
else {resolve("File not Found");}
};
req.send();
});
let result = await promise;
document.getElementById("demo").innerHTML = result;
}
getFile();
浏览器支持
ECMAScript 2017 引入了 JavaScript 关键字 async 和 await。
下表注明了首个完全支持两者的浏览器版本：
| Chrome 55 | Edge 15 | Firefox 52 | Safari 11 | Opera 42 |
| Dec, 2016 | Apr, 2017 | Mar, 2017 | Sep, 2017 | Dec, 2016 |
