Example usage of the new asynquence.map() contrib plugin to update an array against a redis cache

async.js

var transform = function(contents, doneTransform) {
	
	var authExpiry = 60 * 60 * 1000,
		authId = uuid.v4(),
		authKey = 'file:auth:' + authId;
	
	redis.client.set(authKey, JSON.stringify({
		userId: req.user.id
	}));
	redis.client.expire(authKey, authExpiry);
	
	// it was super simple to add a "map" contrib plugin:
	// https://github.com/getify/asynquence/commit/8ff21a2f5275911d645f4b0261be82b6cd758c04
	ASQ().map(contents.files, function(file, done) {
		
		var lookupKey = 'file:path:' + file.path;

		var sq = ASQ();

		// using a nested callback here is exactly the kind of thing
		// promises are meant to alleviate you from. asynquence is all
		// about making promises more palatable, and reducing nested
		// callbacks which lead to fragile "callback hell" inflexibility.
		// in the long run, promises (aka, asynquence sequences) will prove
		// to be more maintainable than callback counter-parts, because
		// sequences can easily have more steps (transformations, etc) added
		// to them later, whereas callbacks require deeper nesting.
		// 
		// in fact, this is the primary reason 'asynquence' is better than
		// 'async', IMO, because it stresses the chained sequencing of promises
		// over one-off async tasks.
		
		// errfcb() provided by the "errfcb" contrib plugin:
		// https://github.com/getify/asynquence/commit/756c65d16930aa421faa855d685085920180fbea
		redis.client.get( lookupKey, sq.errfcb() );

		// for instance, what if later, there was another asynchronous step you
		// needed to do here, before the `file` transformation could take place,
		// like another side look-up, etc? with the callback approach, you're
		// stuck either nesting, or refactoring. with asynquence sequences, just
		// insert a `then(..)` or `gate(..)` or whatever right here, and it "just
		// works". :)

		sq.val(function( fileId ){
			if (fileId) {
				// if we found a fileId, it is already in the cache.
				// convert it to a string and use it.
				fileId = fileId.toString();
			} else {
				// if we didn't, create a new id and cache the file details
				fileId = uuid.v4();
				redis.client.set( 'file:id:' + fileId, JSON.stringify(file) );
				redis.client.set( lookupKey, fileId );
			}
					
			// rewrite the path of the file to {authId}/{fileId}
			file.path = authId + '/' + fileId;

			return file;
		})
		.pipe( done );
	})
	.val(function(files) {
		contents.files = files;
		doneTransform(contents);
	})
	.or(handleError);
	
}