How to use ES8 async/await with streams?

Question

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 1

async/await only works with promises, not with 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` }); 

Now you can await that promise:

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

Answer 2

If you are using node version >= v10.0.0 then you can use stream.pipeline and util.promisify.

const fs = require('fs'); const crypto = require('crypto'); const util = require('util'); const stream = require('stream');  const pipeline = util.promisify(stream.pipeline);  const hash = crypto.createHash('sha1'); hash.setEncoding('hex');  async function run() {   await pipeline(     fs.createReadStream('/some/file/name.txt'),     hash   );   console.log('Pipeline succeeded'); }  run().catch(console.error);