Idiomatic way to wait for multiple callbacks in Node.js

Question

Suppose you need to do some operations that depend on some temp file. Since we\'re talking about Node here, those operations are obviously asynchronous. What is the idiomati

Answer 1

With pure Promises it could be a bit more messy, but if you use Deferred Promises then it's not so bad:

Install:

npm install --save @bitbar/deferred-promise

Modify your code:

const DeferredPromise = require('@bitbar/deferred-promise');

const promises = [
  new DeferredPromise(),
  new DeferredPromise()
];

do_something(tmp_file_name, (err) => {
  if (err) {
    promises[0].reject(err);
  } else {
    promises[0].resolve();
  }
});

do_something_other(tmp_file_name, (err) => {
  if (err) {
    promises[1].reject(err);
  } else {
    promises[1].resolve();
  }
});

Promise.all(promises).then( () => {
  fs.unlink(tmp_file_name);
});

Answer 2

There is no "native" solution, but there are a million flow control libraries for node. You might like Step:

Step(
  function(){
      do_something(tmp_file_name, this.parallel());
      do_something_else(tmp_file_name, this.parallel());
  },
  function(err) {
    if (err) throw err;
    fs.unlink(tmp_file_name);
  }
)

Or, as Michael suggested, counters could be a simpler solution. Take a look at this semaphore mock-up. You'd use it like this:

do_something1(file, queue('myqueue'));
do_something2(file, queue('myqueue'));

queue.done('myqueue', function(){
  fs.unlink(file);
});