Simple Q Promises Node Example

Question

Simple Q Promises Node Example

I'm trying to wrap my head around the Q library/promises in general so I can implement it in my node application, but I'm having trouble finding something concise and specific enough to quickly get started.

Could someone help me translate this code into an asycronous pattern using Q's promise library?

// # For those new to coffeescript

// # '(params) ->' === 'function (params) {}' in coffeescript
// # '@' === 'this' in coffeescript


// # NPM
Q = require 'q'

// # Database class
module.exports = class Database

  constructor: () -> 
    // # mongoose
    @mongoose = require('mongoose')  

    // # Make database connection
    @connect_database()

  connect_database: () ->
    try
      @mongoose.connect('mongodb://127.0.0.1:27017/database')
      return 'Database connected'.green
    catch e
      return ('Database connection error: ' + e.toString()).red

Once I see something directly applicable I think it will be much easier for me to convert the rest of my application to this pattern.

javascript
node.js
coffeescript
promise
q

Answer 1

So basically you want to return the dfd.promise and then resolve or reject it after connecting to the DB.

Q = require 'q'

connect: () ->
 # Create deferred object
 dfd = Q.defer()

 # Attempt to connect
 try
  @mongoose.connect(...)
  # Resolve deferred object
  dfd.resolve('Database connected')

 catch e
  # Reject deferred with error object
  dfd.reject(e)

 # Return promise immediately
 dfd.promise

Now when you run the connect method, you will get a promise object that you can bind to the .then and .fail methods

db.connect()
 .then(msg) ->
 .fail(e) ->