defer.js

function checkGood(good) { return good; }
function checkBad(good) { return !good; }
function checkAlways(good) { return true; }

function defer() {
	var result = null
	var finish = false
	var good
	var handlers = []
	function callHandlers() {
		if (!handlers) return
		for (var i = 0; i < handlers.length; i ++) {
			if (handlers[i].check(good)) {
				handlers[i].fn.apply(null, result)
			}
		}
		handlers = null
	}
	function addHandler(callback, check) {
		if (finish) {
			if (check(good)) callback()
			return this
		}
		handlers.push({ check: check, fn: callback })
		return this
	}
	function handle(status, args) {
		if (finish) return
		finish = true
		good = status
		result = Array.prototype.slice.call(args)
		callHandlers()
	}
	return {
		resolve: function() {
			handle(true, arguments)
		},
		reject: function() {
			handle(false, arguments)
		},
		done: function(callback) {
			addHandler(callback, checkGood)
			return this
		},
		fail: function(callback) {
			addHandler(callback, checkBad)
			return this
		},
		always: function(callback) {
			addHandler(callback, checkAlways)
			return this
		},
		tap: function(fn) {
			fn(this)
			return this
		},
		thru: function(fn) {
			var d = defer()
			this.fail(d.reject)
			this.done(fn(d))
			return d
		},
		pipe: function(fn) {
			return this.thru(function(d) {
				return function() {
					try {
						d.resolve(fn.apply(null, arguments))
					} catch (e) {
						d.reject()
					}
				}
			})
		}
	}
}

defer.callback = function(o) {
	return function(err) {
		if (err) {
			o.reject(err)
		} else {
			o.resolve.apply(o, Array.prototype.slice.call(arguments, 1))
		}
	}
}

module.exports = defer