使用来自bluebird的request.getAsync，如何“管道”到文件

Question

I'm trying to get the content of some pdf's file asynchronously. To do that I'm using Promise.mapSeries with request.getAsync and spread from bluebird .

But in the then I need to get the result of that request using pipe and createWriteStream directly. Something like:

request(url).pipe(fs.createWriteStream(file));

This is the code, I'm using:

const Promise = require('bluebird');
const request = Promise.promisifyAll(require('request'), { multiArgs: true });
const fs = Promise.promisifyAll(require("fs"));

const urls = ['http://localhost/test-pdf/one.pdf', 'http://localhost/test-pdf/two.pdf'];

Promise.mapSeries(urls, url => {
    return request.getAsync({url: url, encoding:'binary'}).spread((response, body) => {
        if (response.statusCode == 200){
            let r = {};
            r.name = url.match(/\/([^/]*)$/)[1]; // get the last part of url (file name)
            r.content = body;
            console.log(`Getting ${r.name}`);
            return r;
        }
        else if (response.statusCode == 404){
            console.log(`The archive ${url.match(/\/([^/]*)$/)[1]} does not exists`);
        }
        else throw new Error(`Unsuccessful attempt. Code: ${response.statusCode}`);
    });
}).then((result) => {
    // Here I want to 'pipe' to a file the result from 'getAsync'
}).catch((error) =>{
    console.error(error);
})

My question:

How can I pipe to a file the result from getAsync using the pipe function ? It is possible?

PD: I know that I can use fs.promises , but just want to know if it's possible to do it in the way I'm posting

Answer 1

I think the answer is already in the question in that .then() seems to be the .pipe() you seek.

What's possibly missing is that (result) should be (results) , ie. an array of all the {name, content} pairs arising from Promise.mapSeries(urls, ...) .

Promise.mapSeries(urls, url => {
    return request.getAsync({'url':url, 'encoding':'binary'}).spread((response, body) => {
        if (response.statusCode == 200) {
            return {
                'name': url.match(/\/([^/]*)$/)[1], // get the last part of url (file name)
                'content': body
            };
        } else if (response.statusCode == 404) {
            throw new Error(`The archive ${url.match(/\/([^/]*)$/)[1]} does not exist`);
        } else {
            throw new Error(`Unsuccessful attempt. Code: ${response.statusCode}`);
        }
    });
}).then((results) => {
    // Here write each `result.content` to file.
}).catch((error) => {
    console.error(error);
});

In practice you would probably not choose to write it that way because every getAsync() would need to complete before any of the writes commenced.

A better flow under most circumstances (and probably the one you want) would be for content from each successful getAsync() to be written as soon as possible :

Promise.mapSeries(urls, url => {
    let name = url.match(/\/([^/]*)$/)[1]; // get the last part of url (file name)
    return request.getAsync({'url':url, 'encoding':'binary'}).spread((response, body) => {
        if (response.statusCode == 200) {
            // write `body.content` to file.
        } else if (response.statusCode == 404) {
            throw new Error(`The archive ${name} does not exist`);
        } else {
            throw new Error(`Unsuccessful attempt. Code: ${response.statusCode}`);
        }
    });
}).catch((error) => {
    console.error(error);
});

Going further, you might choose to handle errors better, for example you may wish to :

• catch individual url/get/write errors
• compile success/failure stats.

Something like this maybe :

Promise.mapSeries(urls, url => {
    let name = url.match(/\/([^/]*)$/)[1] || ''; // get the last part of url (file name)
    if(!name) {
        throw new RangeError(`Error in input data for ${url}`);
    }
    return request.getAsync({'url':url, 'encoding':'binary'}).spread((response, body) => {
        if (response.statusCode == 200) {
            // write `body.content` to file.
            return { name, 'content': body };
        } else if (response.statusCode == 404) {
            throw new Error(`The archive ${name} does not exist`);
        } else {
            throw new Error(`Unsuccessful attempt. Code: ${response.statusCode}`);
        }
    })
    .catch(error => ({ name, error }));
}).then((results) => {
    let successes = results.filter(res => !res.error).length;
    let failures = results.filter(res => !!res.error).length;
    let total = results.length;
    console.log({ successes, failures, total }); // log success/failure stats
}).catch((error) => {
    console.error(error); // just in case some otherwise uncaught error slips through
});