Loop, async/await, createWriteStream, and delay-CodePudding

I am trying to download pdf files from a URL and save them to my local disk. The issue is that I need each file downloaded one at a time with a time delay in between (the delay should be about 1 second). That is to say: after one file is downloaded, the script should wait a full second before downloading the next file).

Wait for the file to be downloaded
Wait for the file to be saved to disk
Wait a full second
Start that loop over again

The Code:

const fs = require('fs');
const fetch = require('node-fetch');

const arrayOfURL = ['www.link.com/file1', 'www.link.com/file2', 'www.link.com/file3'];

const wait = waitTime => new Promise(resolve => setTimeout(resolve, waitTime));

(async() => {

async function downloadAllFiles() {
     return new Promise((resolve, reject) => {
        (async() => {
          for (let i = 0; i < arrayOfURL.length; i  ) {

            console.log(`File ${i} Download Begun`);

            const response = await fetch(arrayOfURL[i]);
            const writeStream = fs.createWriteStream(i   '.pdf');
            response.body.pipe(writeStream);

            async function write() {
                 return new Promise((resolve, reject) => {
                      writeStream.on('finish', function () {
                           resolve('complete');
                      });
                 });
            }

            await write();

            console.log(`----- DOWNLOAD COMPLETE -----`);

            await wait(1000 * i);

            if (i === 2) {
                 resolve('complete');
            }
        } 
      })();
  });
}

await downloadAllFiles();

})();

What I want the logs is to show this:

console.log(`File 0 Download Begun`);
console.log(`File Saved Locally`);
console.log(`----- DOWNLOAD COMPLETE -----`);
***Wait 1 second***

console.log(`File 1 Download Begun`);
console.log(`File Saved Locally`);
console.log(`----- DOWNLOAD COMPLETE -----`);
***Wait 1 second***

console.log(`File 2 Download Begun`);
console.log(`File Saved Locally`);
console.log(`----- DOWNLOAD COMPLETE -----`);
***Wait 1 second***

Instead the logs show this:

console.log(`File 0 Download Begun`);
console.log(`File 1 Download Begun`);
console.log(`File 2 Download Begun`);

console.log(`File Saved Locally`);
console.log(`----- DOWNLOAD COMPLETE -----`);

console.log(`File Saved Locally`);
console.log(`----- DOWNLOAD COMPLETE -----`);

console.log(`File Saved Locally`);
console.log(`----- DOWNLOAD COMPLETE -----`);
***Downloaded at the same time***

And the delay is also not working (all files are downloaded at once) as i in await wait(1000 * i) is always 0;

Any help is appreciated.

CodePudding user response：

The async IIFE is your problem. That IIFE returns a promise that you are doing nothing with, thus the for loop just keeps running because nothing is awaiting that promise that the IIFE returns.

It would be better to fix your code by just removing both IIFEs and removing the new Promise() wrapper entirely like this:

const fs = require('fs');
const fetch = require('node-fetch');

const arrayOfURL = ['www.link.com/file1', 'www.link.com/file2', 'www.link.com/file3'];

const wait = waitTime => new Promise(resolve => setTimeout(resolve, waitTime));

async function downloadAllFiles() {
    for (let i = 0; i < arrayOfURL.length; i  ) {

        console.log(`File ${i} Download Begun`);

        const response = await fetch(arrayOfURL[i]);
        const writeStream = fs.createWriteStream(i   '.pdf');
        response.body.pipe(writeStream);

        async function write() {
            return new Promise((resolve, reject) => {
                writeStream.on('finish', function() {
                    resolve('complete');
                });
                writeStream.on('error', reject);
            });
        }

        await write();

        console.log(`----- DOWNLOAD COMPLETE -----`);

        await wait(1000);
    }
}

downloadAllFiles().then(() => {
    console.log("done");
}).catch(err => {
    console.log(err);
});

And note, I also added an error handler for the writeStream.

When I substitute some real URLs and run this on my system, I get this output and there is a noticeable pause between downloads:

File 0 Download Begun
----- DOWNLOAD COMPLETE -----
File 1 Download Begun
----- DOWNLOAD COMPLETE -----
File 2 Download Begun
----- DOWNLOAD COMPLETE -----
done

async IIFEs can very easily get you in trouble. They return a promise (ALL async functions return a promise) so if you're not paying attention to that promise, then nobody waits for them to finish before continuing on with your code. I never use an async IIFE as there has always (for me) been a cleaner way to write my code.

Also, why are you pausing between loop iterations. That feels like it was an attempt to hack/fix some concurrency issue that should no longer be there or should be fixed a more fundamental way.