Fallback values in promise chains

new-promises-fallback.js

/* UPDATED: This example has been changed to use the new object predicates, that were
 * introduced in Bluebird 3.0. If you are using Bluebird 2.x, you will need to use the
 * older example below, with the predicate function. */

var Promise = require("bluebird");
var fs = Promise.promisifyAll(require("fs"));

Promise.try(function(){
	return fs.readFileAsync("./config.json").then(JSON.parse);
}).catch({code: "ENOENT"}, function(err){
	/* Return an empty object. */
	return {};
}).then(function(config){
	/* `config` now either contains the JSON-parsed configuration file, or an empty object if no configuration file existed. */
});

old-promises-fallback.js

/* This example is ONLY for Bluebird 2.x. When using Bluebird 3.0 or newer, you should
 * use the updated example above instead. */

var Promise = require("bluebird");
var fs = Promise.promisifyAll(require("fs"));

var NonExistentFilePredicate = function(err) {
	return (err.code === "ENOENT");
};

Promise.try(function(){
	return fs.readFileAsync("./config.json").then(JSON.parse);
}).catch(NonExistentFilePredicate, function(err){
	/* Return an empty object. */
	return {};
}).then(function(config){
	/* `config` now either contains the JSON-parsed configuration file, or an empty object if no configuration file existed. */
});