queue.defer()

queue.defer(task[, arguments…])

Adds the specified asynchronous task callback to the queue, with any optional arguments. The task is a function that will be called when the task should start. It is passed the specified optional arguments and an additional callback as the last argument; the callback must be invoked by the task when it finishes. The task must invoke the callback with two arguments: the error, if any, and the result of the task. To return multiple results from a single callback, wrap the results in an object or array.

For example, here’s a task which computes the answer to the ultimate question of life, the universe, and everything after a short delay:

function simpleTask(callback) {
  setTimeout(function() {
    callback(null, {answer: 42});
  }, 250);
}

If the task calls back with an error, any tasks that were scheduled but not yet started will not run. For a serial queue (of concurrency 1), this means that a task will only run if all previous tasks succeed. For a queue with higher concurrency, only the first error that occurs is reported to the await callback, and tasks that were started before the error occurred will continue to run; note, however, that their results will not be reported to the await callback.

Tasks can only be deferred before queue.await or queue.awaitAll is called. If a task is deferred after then, an error is thrown. If the task is not a function, an error is thrown.