co-first

Yield the first async value returned for the co generator library.

Installation

This module is installed via npm:

$ npm install co-first

Background

By default, the co library only supports joining when ALL the asynchronous operations have returned:

var co = require('co');
co(function *() {
  var a = yield asyncFunc1();
  var b = yield asyncFunc1();

  // wait for BOTH a and b to be returned
  var res = yield [a, b];
})();

But it is often useful to be able to just wait for only the FIRST value to be returned, for example, in the event of a timeout:

Example Usage

Returning first caller and value

By default, co-first will return an object that contains both the winning first async operation (on the caller property) and the value on the value property, allowing you to do a switch after the first to work out what happened:

var co = require('co'),
    first = require('co-first');
co(function *() {
  var a = yield asyncFunc1();
  var b = yield asyncFunc1();

  // wait for the FIRST of a and b to be returned
  var firstResult = yield first([a, b]);
  switch (firstResult.caller) {
    case a:
      var valA = firstResult.value;
      break;

    case b:
      var valB = firstResult.value;
      break;
  }
})();

Returning just the first value

When your asynchronous operations return the same value, then you can pass true in for the last variable and just get the first value:

var co = require('co'),
    first = require('co-first');
co(function *() {
  var a = yield asyncFunc1();
  var b = yield asyncFunc1();

  // wait for the FIRST of a and b to be returned and simply return the
  // winning value
  var firstValue = yield first([a, b], true);
})();