How can I fetch an array of URLs with Promise.all?

问题

If I have an array of urls:

var urls = [\'1.txt\', \'2.txt\', \'3.txt\']; // these text files contain \"one\", \"two\", \"three\", respectively.

And I want to build an object that looks like this:

var text = [\'one\', \'two\', \'three\'];

I’ve been trying to learn to do this with fetch, which of course returns Promises.

Some things I’ve tried that don’t work:

var promises = urls.map(url => fetch(url));
var texts = [];
Promise.all(promises)
  .then(results => {
     results.forEach(result => result.text()).then(t => texts.push(t))
  })

This doesn’t look right, and in any case it doesn’t work — I don’t end up with an array [\'one\', \'two\', \'three\'].

Is using Promise.all the right approach here?

回答1:

Yes, Promise.all is the right approach, but you actually need it twice if you want to first fetch all urls and then get all texts from them (which again are promises for the body of the response). So you'd need to do

Promise.all(urls.map(u=>fetch(u))).then(responses =>
    Promise.all(responses.map(res => res.text()))
).then(texts => {
    …
})

Your current code is not working because forEach returns nothing (neither an array nor a promise).

Of course you can simplify that and start with getting the body from each response right after the respective fetch promise fulfilled:

Promise.all(urls.map(url =>
    fetch(url).then(resp => resp.text())
)).then(texts => {
    …
})

回答2:

For some reason neither of Bergi's examples worked for me. It would simply give me empty results. After some debugging it seemes like the promise would return before the fetch had finished, hence the empty results.

However, Benjamin Gruenbaum had an answer here earlier, but deleted it. His method did work for me, so I'll just copy-paste it here, as an alternative in case anyone else runs into any problems with the first solution here.

var promises = urls.map(url => fetch(url).then(y => y.text()));
Promise.all(promises).then(results => {
    // do something with results.
});

回答3:

You should use map instead of forEach:

Promise.all(urls.map(url => fetch(url)))
.then(resp => Promise.all( resp.map(r => r.text()) ))
.then(result => {
    // ...
});

来源：https://stackoverflow.com/questions/31710768/how-can-i-fetch-an-array-of-urls-with-promise-all

标签

javascript

promise

es6-promise

fetch-api