Javascript 答应,一切都过早解决
我想Javascript 答应,一切都过早解决,javascript,es6-promise,Javascript,Es6 Promise,我想获取多个JSON文件。这是我的密码: var timestamps = []; var urls = ['assets/layouts/default.json','assets/layouts/simple.json']; var jsons = []; Promise.all(urls.map(url => { fetch(url) .then(data => data.json()) .then(json => { jsons.push(json)
获取
多个JSON文件。这是我的密码:
var timestamps = [];
var urls = ['assets/layouts/default.json','assets/layouts/simple.json'];
var jsons = [];
Promise.all(urls.map(url => {
fetch(url)
.then(data => data.json())
.then(json => {
jsons.push(json);
timestamps.push(performance.now());
});
}))
.then(() => {
console.log(`All assets loaded. Assets were loaded at: ${timestamps}, this log was made at ${performance.now()}. Loaded data: ${jsons}`);
}
);
问题是,承诺。所有的似乎都过早地解决了。如您所见,我包含了一些调试,它记录了以下内容:
All assets loaded. Assets were loaded at: , this log was made at 37.73. Loaded data:
如果我将console.log
包装在setTimeout
中,并在Promise时节省时间。所有都是这样解决的:
.then(() => {
let promiseResolvedAt = performance.now();
setTimeout(() => {
console.log(`All assets loaded. Assets were loaded at: ${timestamps}, Promise.all was resolved at ${promiseResolvedAt}. Loaded data: ${jsons}`);
})
}
我明白了:
All assets loaded. Assets were loaded at: 63.44000000000001,63.555, Promise.all was resolved at 55.96500000000001. Loaded data: [object Object],[object Object]
我可以通过使用setTimeout
让它像我希望的那样工作,但我确信有更好的方法,我只是做错了一些事情
为什么承诺.all
在其组件承诺解决之前解决
我正在Chrome 55上运行此操作。映射URL数组时,您需要返回调用获取
,否则您的承诺。所有(…)
都在等待[undefined,undefined]
,正如您所观察到的,这将立即解决
Promise.all(urls.map(url => {
return fetch(url)
// ^^^^^^
.then(data => data.json())
.then(json => {
jsons.push(json);
timestamps.push(performance.now());
});
}))
您需要归还承诺:
Promise.all(urls.map(url => {
return fetch(url)
.then(data => data.json())
.then(json => {
jsons.push(json);
timestamps.push(performance.now());
});
}))
发件人:
箭头函数可以有“简明体”或通常的“块”
身体”
在简洁的正文中,只需要一个表达式和一个隐式表达式
附上报税表在块体中,必须使用显式返回
声明
删除fetch
语句周围的{
}
,以允许隐式返回。