Non-blocking condition variables

  • cvar

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1.0.010 years ago10 years agoMinified + gzip package size for cvar in KB



Callback only condition variables


var CondVar = require('cvar');

var cv1 = CondVar();

setTimeout(function(){ cv1.send('asdf') }, 1000);

cv1.recv(function(got) {
    console.log(got); // asdf

// or
cv1(function(got){ console.log(got); /* asdf */ });

var cv2 = CondVar();

for (var ii=0; ii<10; ++ii) {
    setTimeout(function(){ cv2.end() }, 1000 - (ii*100));
cv2.recv(function() {
    console.log('complete'); // after 900 ms


Condition variables are Yet Another way of storing callback state in an async program. They bear some similarlity to promises and continuables.


`var CondVar = require('cvar');

Returns a new condition variable. The optional callback will be called when the cvar is completed, see below. It's the same as calling the recv() method.


  • send(...)

Stores the argument list, emits the complete event. The argument list is passed to the event and to any recv callbacks. The complete event will only be emitted once, even if this is called more then once.
  • recv(callback)

Receives the values sent via send. The argument list passed to send will be passed to the callback when its called. The callback will only ever be called once. As many things can call recv as you want and they'll all receive the same result. Even if the condvar is already complete, the allback is guarenteed to not be executed immediately-- at nextTick at the soonest.
  • begin(groupcallback)

  • end()

The begin method increments an internal counter, the end method decrements it. When the counter reaches zero the groupcallback will be called. If none is specified then send will be called with no arguments. For ecample:
function do_stuff(list_of_hosts) {
    var cv = CondVar();
    var result = {};
    cv.begin(function (){ cv.send(result) });
        ping_host_then_call_callback( host, function () {
            result[host] = ...;

    return cv;

var cv = do_stuff([ ... ]);
The main thing to notice here is the bracketing begin/end calls. These both give you an opportunity to pass in a groupcallback, and to ensure that the CondVar will complete even if listofhosts is an empty array.


  • complete(arguments...)

Used internally. Called when either send() is first called or when the final end() is called.

Prior Art

This module differs from both of these in that it does not support any blocking mode of operation. It's purely callback based. We also do not support a throwing exceptions-- how you pass errors through is up to you. One might adopt the node style of passing an error or null, followed by a value. Alternatively, one might say that if the value is an Error object then something is wrong.