Execute when both(!) events fire .on('end')

Question

I have a node app that reads two files as streams. I use event.on('end') to then work with the results. The problem is I don't really know how I can wait for BOTH events to trigger 'end'.

What I have now is:

    reader1.on('end', function(){
        reader2.on('end',function(){ 
          doSomething();
        });
    }); 

With small files this works, but if one of the files is very large the app aborts.

Answer 1

Your execution logic is somewhat flawed. You ought to do something like this instead

var checklist = [];
 // checklist will contain sort of a counter
function reader_end(){
    if(checklist.length == 2 )
     // doSomething only if both have been added to the checklist
        doSomething();
}
reader1.on('end', function() {
    checklist.push('reader1');
    // increment the counter
    reader_end();
});
reader2.on('end', function() {
    checklist.push('reader2');
    reader_end();
});

Although there are libraries to better handle this sort of stuff, like Async and Promises.

---

With Async you'll need to use compose

var r12_done = async.compose(reader1.on, reader2.on);
r12_done('end', function(){
    doSomething();
});

Edit: I just noticed that since probably reader1.on is a Stream 'end' event which doesn't have the standard callback argument signature of (err, results), this probably won't work. In that case you should just go with Promise.

---

With Promise you'll need to first Promisify and then join

var reader1Promise = Promise.promisify(reader1.on)('end');
var reader2Promise = Promise.promisify(reader2.on)('end');

var reader12Promise = Promise.join(reader1Promise, reader1Promise);

reader12Promise.then(function(){
    doSomething();
});