async声明函数异步执行，遇到await会返回一个promise对象。

这可以取代generator函数，或者说是异步升级版。

async function getStockPriceByName(name) {const symbol = await getStockSymbol(name);const 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);