Javascript 等待循环中调用的所有承诺完成
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Waiting for all promises called in a loop to finish
提问by jkj2000
I'm using the axiospromise library, but my question applies more generally I think. Right now I'm looping over some data and making a single REST call per iteration.
As each call completes I need to add the return value to an object. At a high level, it looks like this:
我正在使用axios承诺库，但我认为我的问题更普遍。现在我正在遍历一些数据并在每次迭代中进行一次 REST 调用。
随着每次调用完成，我需要将返回值添加到一个对象中。在高层次上，它看起来像这样：
var mainObject = {};
myArrayOfData.forEach(function(singleElement){
myUrl = singleElement.webAddress;
axios.get(myUrl)
.then(function(response) {
mainObject[response.identifier] = response.value;
});
});
console.log(convertToStringValue(mainObject));
What's happening of course is when I call console.logthe mainObjectdoesn't have any data in it yet, since axios is still reaching out. What's a good way of dealing with this situation?
这是怎么回事，当然是当我打电话console.log了mainObject没有在其任何数据，因为爱可信还是伸手。处理这种情况的好方法是什么？
Axios does have an allmethod along with a sister spreadone, but they appear to be of use if you know ahead of time how many calls you'll be making, whereas in my case I don't know how many loop iterations there will be.
Axios 确实有一个all方法和一个姊妹方法spread，但是如果您提前知道要进行多少次调用，它们似乎很有用，而在我的情况下，我不知道会有多少次循环迭代。
回答by krasu
You need to collect all of your promises in an array and then use Promise.all:
您需要将所有承诺收集在一个数组中，然后使用Promise.all：
// Example of gathering latest Stack Exchange questions across multiple sites
// Helpers for example
const apiUrl = 'https://api.stackexchange.com/2.2/questions?pagesize=1&order=desc&sort=activity&site=',
sites = ['stackoverflow', 'ubuntu', 'superuser'],
myArrayOfData = sites.map(function (site) {
return {webAddress: apiUrl + site};
});
function convertToStringValue(obj) {
return JSON.stringify(obj, null, '\t');
}
// Original question code
let mainObject = {},
promises = [];
myArrayOfData.forEach(function (singleElement) {
const myUrl = singleElement.webAddress;
promises.push(axios.get(myUrl));
});
Promise.all(promises).then(function (results) {
results.forEach(function (response) {
const question = response.data.items[0];
mainObject[question.question_id] = {
title: question.title,
link: question.link
};
});
console.log(convertToStringValue(mainObject));
});<script src="https://unpkg.com/[email protected]/dist/axios.min.js"></script>It's described in axios docs(Performing multiple concurrent requests section).
它在axios 文档（执行多个并发请求部分）中进行了描述。
Before May 2020 it was possible to do with axios.all(), which is now deprecated.
在 2020 年 5 月之前，可以使用 axios.all()，现在已弃用。
