How to use ES8 async/await with streams?

In https://stackoverflow.com/a/18658613/779159 is an example of how to calculate the md5 of a file using the built-in crypto library and streams.

var fs = require('fs');
var crypto = require('crypto');

// the file you want to get the hash    
var fd = fs.createReadStream('/some/file/name.txt');
var hash = crypto.createHash('sha1');
hash.setEncoding('hex');

fd.on('end', function() {
    hash.end();
    console.log(hash.read()); // the desired sha1sum
});

// read all file and pipe it (write it) to the hash object
fd.pipe(hash);

But is it possible to convert this to using ES8 async/await instead of using the callback as seen above, but while still keeping the efficiency of using streams?

Answer

The await keyword only works on promises, not on streams. There are ideas to make an extra stream-like data type that would get its own syntax, but those are highly experimental if at all and I won’t go into details.

Anyway, your callback is only waiting for the end of the stream, which is a perfect fit for a promise. You’d just have to wrap the stream:

var fd = fs.createReadStream('/some/file/name.txt');
var hash = crypto.createHash('sha1');
hash.setEncoding('hex');
// read all file and pipe it (write it) to the hash object
fd.pipe(hash);

var end = new Promise(function(resolve, reject) {
    hash.on('end', () => resolve(hash.read()));
    fd.on('error', reject); // or something like that. might need to close `hash`
});

There also exists a helper function to do just that in more recent versions of nodejs – pipeline from the stream/promises module:

import { pipeline } from 'node:stream/promises';
const fd = fs.createReadStream('/some/file/name.txt');
const hash = crypto.createHash('sha1');
hash.setEncoding('hex');

// read all file and pipe it (write it) to the hash object
const end = pipeline(fd, hash);

Now you can await that promise:

(async function() {
    let sha1sum = await end;
    console.log(sha1sum);
}());