How to add Promise to event handler in javascript

Question

Now I want to wrap amqp with Promise Q , here are the codes

Sender.prototype.createConnection_ = function () {
    var deferred = Q.defer();
    this.con_ = amqp.createConnection( this.connectOpt_, this.implementOpt_ );
    deferred.resolve( this.con_ );

    return deferred.promise;
}

Sender.prototype.connectionReady_ = function() {
    var deferred = Q.defer(),
      self = this;

    self.con_.on('ready', function() {
        console.log('connection is ok now');
        deferred.resolve(self.con_);
    });
    return deferred.promise;
}

Sender.prototype.createExchange_ = function() {
    var deferred = Q.defer(),
      self = this;

    this.con_.exchange( this.exchangeName_, this.exchangeOpt_, function ( ex ) {
        self.ex_ = ex;
        deferred.resolve(self.ex_);
    });
    return deferred.promise;
}

Sender.prototype.exchangeReady_ = function() {
    var deferred = Q.defer(),
      self = this;

    this.ex_.on('open', function() {
        console.log('Sender: exchange opened');
        deferred.resolve(this.ex_);
    });
    return deferred.promise;
}

Sender.prototype.connect_ = function() {
    var self = this;
    return self.createConnection_()
            .then( self.connectionReady_() )
            .then( self.createExchange_() )
            .then( self.exchangeReady_() )
            .catch( function(err) {
                console.info( err );
            });
}

When I want to invoke connect_ , there is one error show that this.ex_ is null in the exchangeReady_ function.

I want to how to add Q in the event open and ready function ?

Answer 1

You're calling your functions immediately, rather than passing a function reference to the .then() handlers. .then() takes a function reference, not a promise as an argument. Change to this:

Sender.prototype.connect_ = function() {
    return this.createConnection_()
            .then( this.connectionReady_.bind(this) )
            .then( this.createExchange_.bind(this) )
            .then( this.exchangeReady_.bind(this) )
            .catch( function(err) {
                console.info( err );
            });
}

The .bind(this) lets you pass a function reference (something the .then() infrastructure can call later) and still have it bound to this .

---

It looks like you may also have binding issues when you pass callbacks like this:

amqp.createConnection( this.connectOpt_, this.implementOpt_ );

These callbacks will not stay bound to this . Instead, use .bind() like this on any callback that is method:

amqp.createConnection( this.connectOpt_.bind(this), this.implementOpt_.bind(this) );

The same issue exists several other places in your code.