CODEWITHSUNDEEP
javascriptunderscore.jsasync.js
Chintan Pathak
Chintan Pathak

Reputation: 301

What is the difference between the async.compose function and underscore.compose function?

Both seem to be doing the same thing, i.e. calculating f(g(h(x))), when called like this async.compose(f, g, h) or _.compose(f, g, h).

Is there any difference between the two calls ?

Upvotes: 1

Views: 1050

Answers (2)

1111161171159459134
1111161171159459134

Reputation: 1215

I faced same situation where I needed one function I got from some lib to use the return value from async.compose() call. So I did wonder: How can one get the return value from async.compose()?
If you write:

var gotFromAsyncCompose = async.compose(
  function(gotParam, callback) {
    callback(null, 'hi ' + gotParam + '!';
  },
  function(name, callback) {
    callback(null, name.toUpperCase());
  }
);
var returnValue = gotFromAsyncCompose('moe', function(err, result){
  return result;
});
console.log(returnValue);

You'll get undefined as the value of returnValue.
So, async.compose() always return undefined as return value even if the last executed statement is return 'A';. You'll still get undefined!

When it comes to _.compose(), I tried the same:

var w = _.compose(
  function(name){
    return 'hi '+name;
  },
  function(got){
    return got.toUpperCase() +'!';
  });
);
var returnValue = w('moe');
console.log(returnValue);

This time, you get 'hi MOE!' as expected as the value of returnValue. You don't get undefined like the behaviour of async.compose().

I didn't wrote any of these compose functions, but it seems that the difference between the two seems to be that async.compose() won't return any value whereas _.compose() will return the value you pass to return within the last executed function.
But, there is another difference: _.compose() doesn't take care of any asynchronous call you make inside your functions. You have to write only synchronous code within all the functions that comprise your _.compose().

The four main differences I can see as of April 28th 2014 are:

  • async.compose() does not return a value
  • async.compose() does take care of asynchronous calls within functions
  • _.compose() does return a value
  • _.compose() does not take care of asynchronous calls within functions

Upvotes: 1

SLaks
SLaks

Reputation: 887547

As its name implies, async.compose composes asynchronous functions.

It gets the result via callback parameter, not return value.

Upvotes: 3

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