Javascript 如何获得[[承诺价值]]';如果.then()不';不行吗?
情况是:我有一个Javascript 如何获得[[承诺价值]]';如果.then()不';不行吗?,javascript,reactjs,promise,Javascript,Reactjs,Promise,情况是:我有一个城市数组,函数getCityForecast,它获取每个城市的预测,并将收到的转换数据返回到所需的格式。我需要将它们保存到数组中,并将此数组保存在组件的状态中 this.state = { cities: [ 'Санкт-Петербург', 'Москва', 'Казань', 'Самара', 'Красноярск', 'Чита', 'Чел
城市
数组,函数getCityForecast
,它获取每个城市的预测,并将收到的转换数据返回到所需的格式。我需要将它们保存到数组中,并将此数组保存在组件的状态中
this.state = {
cities: [
'Санкт-Петербург',
'Москва',
'Казань',
'Самара',
'Красноярск',
'Чита',
'Челябинск',
'Оренбург',
'Ростов-на-Дону',
'Орск'
],
cityObjects: []
};
this.getCityForecast = this.getCityForecast.bind(this);
this.renderCities = this.renderCities.bind(this);
}
getForecastData = async (cityKey, cityName) => {
const apiKey = 'ExAmPlEkEy';
const forecastUri = 'http://dataservice.accuweather.com/forecasts/v1/daily/1day/';
const uriToFetch = `${forecastUri}${cityKey}?language=ru&metric=true&details=true&apikey=${apiKey}`;
try {
let response = await fetch(uriToFetch);
if(response.ok) {
let jsonResponse = await response.json(); // Converts into JSON
if (jsonResponse.DailyForecasts) {
let cityData = jsonResponse.DailyForecasts.map(forecast => ({ //Data converted here into a convenient object
icon: forecast.Day.Icon,
iconPhrase: forecast.Day.IconPhrase,
tempValue: forecast.Temperature.Maximum.Value,
tempUnit: forecast.Temperature.Maximum.Unit,
cityKey: cityKey,
cityName: cityName
})
);
let renderCity = cityData.map(city => ( // Presented in React.js manner
<div
className="weather"
key={city.cityKey}>
<h1>{city.cityName}</h1>
<img
src={`http://apidev.accuweather.com/developers/Media/Default/WeatherIcons/${city.icon}-s.png`}
alt={city.iconPhrase}
className="weathericon" />
<h2>{`Температура: ${city.tempValue}°${city.tempUnit}`}</h2>
</div>
)
);
return renderCity; // Retuns a formatted city forecast
} else {
return [];
}
}
throw new Error('Forecast request failed!');
} catch (error) {
console.log(error);
}
}
renderCities = () => { // applies this.getCityForecast() to the array of city names
if(this.state.cities) {
const cityObj = Promise.all(this.state.cities
.map(city => this.getCityForecast(city)))
.then(promiseResp => (promiseResp)) // CASE ONE
/*.then(val => (val))*/ // CASE TWO <-- Not the above promise's value
.catch(e => {console.log(e)});
console.log(cityObj); // Save response to this.cityObjects
}
}
在案例二中,我得到:
__proto__: Promise
[[PromiseStatus]]: "pending"
[[PromiseValue]]: undefined
如何获得案例一的[[PromiseValue]]
内容?您应该将呈现
设置为异步函数,并等待承诺:
renderCities = async () => { // applies this.getCityForecast() to the array of city names
if(this.state.cities) {
const cityObj = await Promise.all(this.state.cities
.map(city => this.getCityForecast(city)))
.catch(e => {console.log(e)});
console.log(cityObj); // Save response to this.cityObjects
}
}
.then(promiseResp=>(promiseResp))
或。then(val=>(val))
实际上什么都不做–您只是将承诺中的值映射到它本身
另请参见“如果<代码>那么<代码>不起作用”是什么意思?您所处的情况有点不清楚。我认为稍微重构一下代码不会有什么坏处。首先,让我们将getForecastData
作为一个函数,它实际返回获取的数据,而不返回任何其他内容,并将react/jsx代码从它移动到一个新函数,该函数可能称为renderCities
。然后我假设您可以将城市数据存储在状态中,并使用renderCities
?相应地渲染它。然后()
处理程序以级联方式执行,它们与switch
语句的情况不同。一个.then()
处理程序返回的值是下一个.then()
处理程序的参数.then(promiseResp=>(promiseResp))
是不可操作的。它只返回它接收到的值。Super noobwait
/async
问题。。。您需要在这里的某个地方使用返回按钮吗?或者是自动处理的?@apsillersrenderCities
创建了一个无条件的承诺(undefined
),它不会返回任何东西。传递给map
的arrow函数有其通常的隐式返回。
renderCities = async () => { // applies this.getCityForecast() to the array of city names
if(this.state.cities) {
const cityObj = await Promise.all(this.state.cities
.map(city => this.getCityForecast(city)))
.catch(e => {console.log(e)});
console.log(cityObj); // Save response to this.cityObjects
}
}