Is it possible to return values from asynchronous functions in node.js?

Question

I'm likely reinventing the wheel here, but I want to make a function that takes a filename and returns the base64 encoded string asynchronously.

I'm really only familiar with callbacks, but trying to understand promises as well. My initial callback function looks like this --

blob2Base64callback.js

module.exports = {
  blob2base64 : function(file) {
    fs.readFile(file, 'utf8', function(err, contents){
      if (err) {
        console.log('file not found');
        return '';
      } else {
        var base64str = Buffer.from(contents).toString('base64');
        console.log(base64str.length);
        return base64str;
      }
    });

  }, //..other functions
};

This callback doesn't work because I'm not actually returning something from the blob2base4 function and if I modify this to something like the following:

module.exports = {
  blob2base64 : function(file) {
    return fs.readFile(file, 'utf8', function(err, contents){
      //same implementation as above, cut to improve readability
    });

  }
};

It returns undefined because it doesn't wait to be executed. As far as I know, there is no possible way to return something to blob2base64 using callbacks.

So I looked into promises and tried the following:

blob2Base64promise.js

blob2base64 : function(file) {
  console.log('function called');
  const getFile = new Promise(function(resolve, reject){

    fs.readFile(file, 'utf8', function(err, contents){
      if (err) {
        console.log('file not found');
        reject(err)
      } else {
        //console.log(contents);
        resolve(contents);
      }
    });

  }).then(contents =>{
    var base64str = Buffer.from(contents).toString('base64');
    return base64str;
  }).catch(err => console.log(err));
}

Is there anyway I can return the promise to the function and extract the resolve portion to get base64str? Am I right in thinking, this behavior is only possible with promises, but not callbacks. Or is it not possible with either syntax?

Also, I thought callbacks and promises couldn't be used together, yet this promise is just a wrapper around a callback.

@Pointy how do I extract the base64str from the returned promise? — ryanmattscott
– ryanmattscott, Commented Jun 28, 2018 at 13:37
The calling environment can add another .then() callback to the returned Promise value. — Pointy
– Pointy, Commented Jun 28, 2018 at 13:38
If that seems onerous, you can start looking into async and await, which basically are ways of wrapping up Promise callback code in a less clumsy syntax. — Pointy
– Pointy, Commented Jun 28, 2018 at 13:39
Thanks I'll look into that, and just for anyone else that looks at this later I was able to return it using parse.blob2base64('file.pdf').then(val=> console.log(val)); — ryanmattscott
– ryanmattscott, Commented Jun 28, 2018 at 13:47

clay · Accepted Answer · 2018-06-28 13:48:45Z

(Answering in reverse order)

Typically callbacks and Promises are not mixed, just because it's messy. They are possible to intertwine, if necessary. There are utilities to turn callbacks into Promises ("promisify"), and Promises are becoming much more common, especially with the async/await syntax of ES6.

For Promises, be sure to return the Promise itself. In your example, return getFile. What the Promise chain eventually returns you must await. That could be the async/await keywords in ES6. You're call could be:

async function() { let result = await blob2base64('myFile.png') }

Which is just a nicer syntax for knowing your are returning a Promise. You could also do it like this:

function () {
  blob2base64('myFile.png')
  .then( result => {
    // use result here
  })
  // you could catch as well.
}

For callbacks, you need to also pass a callback to your function. That is what makes it asynchronous and where you give your results:

blob2base64 : function(file, callback) {
  fs.readFile(file, 'utf8', function(err, contents){
    if (err) {
      console.log('file not found');
      callback(err)
    } else {
      var base64str = Buffer.from(contents).toString('base64');
      console.log(base64str.length);
      callback(null, base64str)
    }
  });

And you would call it like: blob2base64( 'myFile.png', function (err,result) { })

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