promises.md

promises

http://github.com/bengl/forwardjs-promise-talk terminal font - Perfect DOS VGA 437

• has reject function
• has fulfill function
• returns an object that handles
  • .then(function(value){});
  • .then(function(result){});
    • .then() returns promises
  • .catch(function(err){});

issues

• built-in node constructs use callbacks, not promises
• modules don't reliably use promises

solutions

• bluebird
  • install and require
• hue
• in es6 draft (not reliably on spec)

using bluebird

$ npm i --save bluebird

var Promise = require('bluebird');
var fs = require('fs');
Promise.promisfyAll(fs);
readFile('stuff.json', {encoding: 'utf8'})
.then(JSON.parse)
.then(function(stuff){
	console.log(stuff.thing);
});

• promise libaries can be resource intensive
• leverage at init, avoid in actual process flow

backwards compatibility

• API considerations
• very simple (not necessarily realistic)

function getUser(someArt){
	return getFromDBAsync(someArg)
	.then(function(data){
		return getFromotherDBAsync(data);
	}).nodeify(cb);
}

• hook function

function getUser(hook, cb){
	//...
	return safelyPromisify(hook)()
	.then()
}

again, code needs to be minimal, step through

avoiding callback hell

async.parallel([
	function(next){
		obj.doSomething1(next);
	},
	function(next){
		obj.doSomething2(next);
	}
]);

• in promise format

Promise.all([
	obj.doSomething1Async(),
	obj.doSomething2Async()
])

• waterfall

async.waterfall([
	function(next){
		obj.doSomething1(next);
	},
	function(next){
		obj.doSomething2(somevalue, next);
	}
])

• in promise format

Promise.all(obj.doSomething1Async().then(function(data){
	obj.doSomething2(somevalue);
}));

• whilst

function whilst(condition, aciton){
	return (function loop(){
		if (condition()){
			return action().then(loop);
		}
	})
})();

in practice
-----------
- Thrush library github.com/DeNA/thrush

Promise.safelyPromisify Promise.series Promise.whilst

edge cases
----------
- spread 
	- raises value to higher scope so it is available to all calls
	- can be called with config obj of {spread:true}

stack traces
------------
- bluebird adds a lot of stack frames
- adds "From previous event"
	- Promise.longStackTraces();

testing
-------
- using mocha < 1.18.0
	- have to call done at end of callback
- newer mocha, can return promise
- 

worth it?
---------