deferred-open

Make methods wait for async constructors.

For example, database libraries like LevelUp or node-redis expose an API that you can use immediately, although the connection hasn't been established yet. This greatly helps reduce callback depth.

In most cases you rather want this:

var db = levelup('/db');
db.get('foo', function (err, val) {
  val == 'bar';
});

than this:

levelup('/db', function (err, db) {
  if (err) throw err;
  db.get('foo', function (err, val) {
    val == 'bar';
  })
});

So, all calls to member functions have to be deffered until the connection is up. This module helps by mixing in to an Object and deferring operations until the constructor is done doing its async stuff.

Usage

Install Deferred onto a function and make doAThing wait until the constructor finished doing its async operations.

var Deferred = require('deferred-open');
 
function Something () {
  var self = this;
 
  Deferred.install(self); // => self._ready, self._queue
 
  self.loaded = false; // fake internal state
 
  setTimeout(function () { // some async function
    self.loaded = true;
    self._ready();
  }, 500);
}
 
Something.prototype.doAThing = Deferred(function () {
  if (!this.loaded) throw new Error('oops');
  console.log('success');
});
 

And use it like this:

var something = new Something();
something.doAThing(); // "success" after 500ms

Non deferred open

If you want to support non deferred open too, use this._queue:

function Something (cb) {
  var self = this;
 
  if (!(self instanceof Something)) return new Something(cb);
  Deferred.install(self);
 
  if (cb) self._queue(cb); // allow async api too
 
  self.loaded = false;
 
  setTimeout(function () {
    self.loaded = true;
    self._ready();
  }, 500);
}

Now you can use it like this:

// Deferred
var something = new Something();
something.doAThing();
 
// Or non deferred
Something(function (err, something) {
  something.doAThing();
});

API

Deferred.install(fn)

Call this in fn's constructor in order to install the _ready and _queue member functions.

this._ready([err])

Call this when you finished doing your async stuff.

If you pass something as the err parameter,

• a deferred call will throw
• a non deferred call will pass that as first argument to the callback

this._queue(fn)

Call fn with (err, this) as soon as this._ready() has been called (with the optional error argument).

Deferred(fn)

Returns a function that gets called only after the internal deferred object has been signaled by this._ready(). Returns this when not ready, so it works with chained/fluent apis.

Deferred.stream(fn[, opts])

Returns a function the gets called only after the constructor is ready, but returns a paused stream immediately, that will later be hooked up with the real stream.

opts is an optional options object that supports

• readable (Boolean): The returned stream should be readable. Default: true.
• writable (Boolean): The returned stream should be writable. Default: true.

Installation

With npm do

$ npm install deferred-open

License

(MIT)

Copyright (c) 2013 Julian Gruber <julian@juliangruber.com>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.